Optional Scalars support for Rust (#6034)

* First draft of rust optionals

* Code cleanup around ftBool and ftVectorOfBool

* Tests for Rust optional scalars

* test bools too

Co-authored-by: Casper Neo <cneo@google.com>

src/idl_parser.cpp

@@ -2258,7 +2258,7 @@ CheckedError Parser::CheckClash(std::vector<FieldDef *> &fields,
 
 
 bool Parser::SupportsNullableScalars() const {
-  return opts.lang_to_generate == 0;  // No support yet.
+  return !(opts.lang_to_generate & ~IDLOptions::kRust);
 }
 
 bool Parser::SupportsAdvancedUnionFeatures() const {