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()`.

src/idl_gen_rust.cpp

@@ -1310,6 +1310,7 @@ class RustGenerator : public BaseGenerator {
       auto u = field.value.type.enum_def;
 
       code_.SetValue("FIELD_NAME", Name(field));
+      code_.SetValue("FIELD_TYPE_FIELD_NAME", field.name);
 
       for (auto u_it = u->Vals().begin(); u_it != u->Vals().end(); ++u_it) {
         auto &ev = **u_it;
@@ -1328,7 +1329,7 @@ class RustGenerator : public BaseGenerator {
         code_ += "  #[allow(non_snake_case)]";
         code_ += "  pub fn {{FIELD_NAME}}_as_{{U_ELEMENT_NAME}}(&self) -> "
                  "Option<{{U_ELEMENT_TABLE_TYPE}}<'a>> {";
-        code_ += "    if self.{{FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
+        code_ += "    if self.{{FIELD_TYPE_FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
         code_ += "      self.{{FIELD_NAME}}().map(|u| "
                  "{{U_ELEMENT_TABLE_TYPE}}::init_from_table(u))";
         code_ += "    } else {";