Module convert

Conversion of internal Rust compiler items to stable ones.

Modules

• abi 🔒
  Conversion of internal Rust compiler rustc_target and rustc_abi items to stable ones.
• error 🔒
  Handle the conversion of different internal errors into a stable version.
• mir 🔒
  Conversion of internal Rust compiler mir items to stable ones.
• ty 🔒
  Conversion of internal Rust compiler ty items to stable ones.