* 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 d554d79fecf5afd6da6fb993b30b4cd523a5889a.

* 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.