Type names in generated Rust code can't contain hyphens

[Michael-F-Bryan]

@meelislootus this fixes your issue where the noise-filtering proc block has a hyphen and our rune-codegen tries using that as the proc block's namespace.

Meeliss-MacBook-Pro-3:rune meelislootus$ ./target/debug/rune build ./examples/microspeech/Runefile 
[2021-05-07T14:28:38.890Z DEBUG rune::build] Parsing "./examples/microspeech/Runefile"
[2021-05-07T14:28:38.891Z DEBUG rune::build] Compiling microspeech in "/Users/meelislootus/Library/Caches/runes/microspeech"
[2021-05-07T14:28:38.913Z DEBUG rune_codegen] Executing "cargo" "+nightly" "build" "--target=wasm32-unknown-unknown" "--quiet" "--release"
error[E0425]: cannot find value `noise` in this scope
 --> lib.rs:22:31
   |
22 |   let mut noise_filtering = noise-filtering::NoiseFiltering::default();
   |                ^^^^^ not found in this scopeerror[E0433]: failed to resolve: use of undeclared crate or module `filtering`
 --> lib.rs:22:48
   |
22 |   let mut noise_filtering = noise-filtering::NoiseFiltering::default();
   |                         ^^^^^^^^^^^^^^ not found in `filtering`
   |
help: consider importing this struct
   |
8  | use noise_filtering::NoiseFiltering;
   |error: aborting due to 2 previous errorsSome errors have detailed explanations: E0425, E0433.
For more information about an error, try `rustc --explain E0425`.
error: could not compile `microspeech`To learn more, run the command again with --verbose.
Error: Rune compilation failedCaused by:
  Compilation failed

Closes #145.