Added initial Rust codegen-meta implementation. (#403)

* Added initial Rust codegen-meta implementation.

* Replace 'Cretonne' in comments.

* Prevent iterator overflow.

* 1.25.0 compatibility changes.

* Implemented debug traits for type variants.

* Added consistent comments.

* Cleaned up a loop via clippy fix.

* Added new license to codegen-meta Cargo.toml

* Edited lane type iterator `next` method.

* Removed functions that are not needed in Rust, and edited desc.

* Debug trait derived for valuetype.

* Added comments for iterator types in the base types submodule.

* Numbering is now handled in the cdsl/types.rs file.

* Moved type number logic into cdsl/types.

* Repeating the lane change cleanup.

* Removed codegen-meta crate from codegen deps.

* Typo fix.

* Addressing a patch note.

* Addressing patch note.

* Lowercase in vector names.

* Fixing a comment bug.

* Added a copy of the license file.

* Formatting changes.

* Cleaned up the vector type numbering.

* 1.25 compatibility.

* Fixed pattern match arms.

lib/codegen/Cargo.toml

@@ -21,6 +21,9 @@ target-lexicon = { version = "0.0.3", default-features = false }
 # machine code. Integration tests that need external dependencies can be
 # accomodated in `tests`.
 
+[build-dependencies]
+cranelift-codegen-meta = { path = "../codegen-meta", version = "0.15.0" }
+
 [features]
 # The "std" feature enables use of libstd. The "core" feature enables use
 # of some minimal std-like replacement libraries. At least one of these two