Support building Heapster shapes from Rust type declarations

[ChrisEPhifer]

This PR adds a number of RsConvert instances and a simple parser entry-point to support the synthesis of Heapster shapes from Rust types.

Marked as WIP as I implement the saw-script side of things (adding a command to interface with these features); there is a branch off wip-heapster for that work, named the same as this one (feature/rust-type-decls).

[ChrisEPhifer]

@eddywestbrook I believe this is ready to go! All of the examples in rust_data.saw have been updated to use this new machinery, and the Coq output looks to be error-free.

Once we get this merged I'll tag you in saw-script for the corresponding merge into wip-heapster.

[eddywestbrook]

A few nitpicky things:

1. Please add a Haddock to isRecursiveDef
2. Maybe change the Haddock for parseLLVMShapeFromRust to say that it is parsing a type definition or declaration or whatever you think the correct noun is. It's a bit confusing (at least to me) to say it is parsing types, because other code in the same module does in fact parse types to shapes
3. And on a similar note, maybe change the name of parseLLVMShapeFromRust to something like parseNamedShapeFromRustDecl or parseRustTypeDecl or rsParseConvertTypeDecl or something like that

[ChrisEPhifer]

Nits have been picked :) Gonna merge this in!