Optional Scalars support for Rust

[CasperN]

Context on Optional Scalars: #6014. This CL enables optional scalars in Rust. The schema

table Monster { mana: int = null; }

generates code like

impl Monster {
  fn mana() -> Option<i32>;
}

and analogously for other scalars. See tests/optional_scalars_generated.rs for a detailed example. In addition to modifying idl_gen_rust.cc to support optional scalars, I also cleaned up the code a bit:

• I removed the special casing offtBool and ftVectorBool. This lead to a minor whitespace change in the generated code, I left a TODO to fix that in a future CL.
• I deleted the GetDefaultConstant function that seems to do nothing.
• Removed an unused #[macro_use] in Flexbuffers.

Props to @rw for writing the Rust Flabuffers library in such a way that no changes were needed for this feature.

[aardappel]

Generally looks good to me, and a good template (and schema) to follow for other languages.

@rw can comment on Rust specifics :)

[CasperN]

Bump @rw

[rw]

@CasperN LGTM! Thanks for cleaning up some of the conditionals in the generator, too.