[rust] Make enum names public (#5690)

* 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.

* Address comments raised in PR

* Update documentation comments per feedback

* Fix typo

* [rust] Make enum variant names public.

* Update generated test files

* Add test for public enum names

tests/namespace_test/namespace_test1_generated.rs

@@ -77,7 +77,7 @@ pub const ENUM_VALUES_ENUM_IN_NESTED_NS:[EnumInNestedNS; 3] = [
 ];
 
 #[allow(non_camel_case_types)]
-const ENUM_NAMES_ENUM_IN_NESTED_NS:[&'static str; 3] = [
+pub const ENUM_NAMES_ENUM_IN_NESTED_NS:[&'static str; 3] = [
     "A",
     "B",
     "C"