[rust] Add use declarations to Rust-generated bindings for imported FB definitions (#5645)

* Bugfix for Rust generation of union fields named with language keywords

Looking at ParseField, it appears that in the case of unions, an extra field with a `UnionTypeFieldSuffix` is added to the type definition, however, if the name of this field is a keyword in the target language, it isn't escaped.

For example, if generating code for rust for a union field named `type`, flatc will generate a (non-keyword escaped) field named `type_type` for this hidden union field, and one (keyword escaped) called `type_` for the actual union contents.

When the union accessors are generated, they refer to this `type_type` field, but they will escape it mistakenly, generating code like this:

```
  #[inline]
  #[allow(non_snake_case)]
  pub fn type__as_int(&self) -> Option<Int<'a>> {
    if self.type__type() == Type::Int {
      self.type_().map(|u| Int::init_from_table(u))
    } else {
      None
    }
  }
```

Which will fail to build because the field is called `self.type_type()`, not `self.type__type()`.

* [Rust] Add crate-relative use statements for FBS includes.

At present if a flatbuffer description includes a reference to a type in
another file, the generated Rust code needs to be hand-modified to add
the appropriate `use` statements.

This assumes that the dependencies are built into the same crate, which
I think is a reasonable assumption?

* Revert "[Rust] Add crate-relative use statements for FBS includes."

This reverts commit d554d79fec.

* Add updated generated test files.

* Fixing Rust test harness to handle new includes.

Test binaries need to add references to generated code that's
transitively included.

This also has the knock-on in that this code (which is referenced by
include directives directly in the flatbuffer schema files) also needs
to be generated, hence the changes to generate_code.sh.

* Test harnesses expect test data to be checked in.

Put include_test2 files into the same directory as the include_test2
schema definition.

Update all code generation scripts (forgot the batch file from last
time).

Path updates in Rust test.

* Include updated generated code

* Address comments raised in PR

* Fix failing Rust tests.

* Previous merge clobbered this branch change.

* Add updated imports to benchmarks.

* Clarifying comment per PR request

* Update documentation comments per feedback

* Remove non-Rust generated files for include tests, per feedback from @rw/@aardappel

* Broken code generation batch file

* Fix typo

* Add TODO for tidying up use declaration traversal sometime in the future

* Update test files.

tests/include_test/include_test1_generated.rs

@@ -0,0 +1,87 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+
+
+use crate::include_test2_generated::*;
+use std::mem;
+use std::cmp::Ordering;
+
+extern crate flatbuffers;
+use self::flatbuffers::EndianScalar;
+
+pub enum TableAOffset {}
+#[derive(Copy, Clone, Debug, PartialEq)]
+
+pub struct TableA<'a> {
+  pub _tab: flatbuffers::Table<'a>,
+}
+
+impl<'a> flatbuffers::Follow<'a> for TableA<'a> {
+    type Inner = TableA<'a>;
+    #[inline]
+    fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+        Self {
+            _tab: flatbuffers::Table { buf: buf, loc: loc },
+        }
+    }
+}
+
+impl<'a> TableA<'a> {
+    #[inline]
+    pub fn init_from_table(table: flatbuffers::Table<'a>) -> Self {
+        TableA {
+            _tab: table,
+        }
+    }
+    #[allow(unused_mut)]
+    pub fn create<'bldr: 'args, 'args: 'mut_bldr, 'mut_bldr>(
+        _fbb: &'mut_bldr mut flatbuffers::FlatBufferBuilder<'bldr>,
+        args: &'args TableAArgs<'args>) -> flatbuffers::WIPOffset<TableA<'bldr>> {
+      let mut builder = TableABuilder::new(_fbb);
+      if let Some(x) = args.b { builder.add_b(x); }
+      builder.finish()
+    }
+
+    pub const VT_B: flatbuffers::VOffsetT = 4;
+
+  #[inline]
+  pub fn b(&self) -> Option<my_game::other_name_space::TableB<'a>> {
+    self._tab.get::<flatbuffers::ForwardsUOffset<my_game::other_name_space::TableB<'a>>>(TableA::VT_B, None)
+  }
+}
+
+pub struct TableAArgs<'a> {
+    pub b: Option<flatbuffers::WIPOffset<my_game::other_name_space::TableB<'a >>>,
+}
+impl<'a> Default for TableAArgs<'a> {
+    #[inline]
+    fn default() -> Self {
+        TableAArgs {
+            b: None,
+        }
+    }
+}
+pub struct TableABuilder<'a: 'b, 'b> {
+  fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
+  start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,
+}
+impl<'a: 'b, 'b> TableABuilder<'a, 'b> {
+  #[inline]
+  pub fn add_b(&mut self, b: flatbuffers::WIPOffset<my_game::other_name_space::TableB<'b >>) {
+    self.fbb_.push_slot_always::<flatbuffers::WIPOffset<my_game::other_name_space::TableB>>(TableA::VT_B, b);
+  }
+  #[inline]
+  pub fn new(_fbb: &'b mut flatbuffers::FlatBufferBuilder<'a>) -> TableABuilder<'a, 'b> {
+    let start = _fbb.start_table();
+    TableABuilder {
+      fbb_: _fbb,
+      start_: start,
+    }
+  }
+  #[inline]
+  pub fn finish(self) -> flatbuffers::WIPOffset<TableA<'a>> {
+    let o = self.fbb_.end_table(self.start_);
+    flatbuffers::WIPOffset::new(o.value())
+  }
+}
+