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-meta/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "cranelift-codegen-meta"
+authors = ["The Cranelift Project Developers"]
+version = "0.15.0"
+description = "Metaprogram for cranelift-codegen code generator library"
+license = "Apache-2.0 WITH LLVM-exception"
+documentation = "https://cranelift.readthedocs.io/"
+repository = "https://github.com/CraneStation/cranelift"
+keywords = ["compile", "compiler", "jit"]
+
+[dependencies]
+# It is a goal of the cranelift-codegen crate to have minimal external dependencies.
+# Please don't add any unless they are essential to the task of creating binary
+# machine code.
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "CraneStation/cranelift" }