Make use of associated constants

[neachdainn]

There was a bit of a hack to handle constants in the LCM definition files. All of that is moot now that Rust 1.20 has implemented associated constants. This should simplify the mod structure of the generated Rust and be more like what the C++ version.

[neachdainn]

Another thing that could be done is make the hash an associated constant. I'm not 100% on whether or not the hash function is called often enough to make that change worth the effort, but it is possible and I don't see any downsides.

[adeschamps]

I definitely agree. Even though I expect the compiler would inline those functions, they really should be constants now that that feature is available.