[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/rust_usage_test/tests/integration_test.rs

@@ -245,6 +245,16 @@ mod generated_constants {
             my_game::example::Color::Green,
             my_game::example::Color::Blue,
         ]);
+        assert_eq!(my_game::example::ENUM_NAMES_COLOR, [
+            "Red",
+            "Green",
+            "",
+            "",
+            "",
+            "",
+            "",
+            "Blue"
+        ]);
 
         assert_eq!(-1, my_game::example::ENUM_MIN_RACE);
         assert_eq!(2, my_game::example::ENUM_MAX_RACE);
@@ -254,6 +264,12 @@ mod generated_constants {
             my_game::example::Race::Dwarf,
             my_game::example::Race::Elf,
         ]);
+        assert_eq!(my_game::example::ENUM_NAMES_RACE, [
+            "None",
+            "Human",
+            "Dwarf",
+            "Elf"
+        ]);
 
         assert_eq!(0, my_game::example::ENUM_MIN_ANY);
         assert_eq!(3, my_game::example::ENUM_MAX_ANY);
@@ -263,6 +279,12 @@ mod generated_constants {
             my_game::example::Any::TestSimpleTableWithEnum,
             my_game::example::Any::MyGame_Example2_Monster,
         ]);
+        assert_eq!(my_game::example::ENUM_NAMES_ANY, [
+            "NONE",
+            "Monster",
+            "TestSimpleTableWithEnum",
+            "MyGame_Example2_Monster"
+        ]);
 
         assert_eq!(0, my_game::example::ENUM_MIN_ANY_UNIQUE_ALIASES);
         assert_eq!(3, my_game::example::ENUM_MAX_ANY_UNIQUE_ALIASES);
@@ -272,6 +294,12 @@ mod generated_constants {
             my_game::example::AnyUniqueAliases::TS,
             my_game::example::AnyUniqueAliases::M2,
         ]);
+        assert_eq!(my_game::example::ENUM_NAMES_ANY_UNIQUE_ALIASES, [
+            "NONE",
+            "M",
+            "TS",
+            "M2"
+        ]);
 
         assert_eq!(0, my_game::example::ENUM_MIN_ANY_AMBIGUOUS_ALIASES);
         assert_eq!(3, my_game::example::ENUM_MAX_ANY_AMBIGUOUS_ALIASES);
@@ -281,6 +309,12 @@ mod generated_constants {
             my_game::example::AnyAmbiguousAliases::M2,
             my_game::example::AnyAmbiguousAliases::M3,
         ]);
+        assert_eq!(my_game::example::ENUM_NAMES_ANY_AMBIGUOUS_ALIASES, [
+            "NONE",
+            "M1",
+            "M2",
+            "M3"
+        ]);
     }
 }