Sorbit derive macro implementation

This crate contains the implementation of the derive macros used by sorbit, and is not meant to be used on its own.

This crate is split out from the sorbit_derive crate, but isn't itself a procedural macro crate. The only reasons for this split is because procedural macro crates behave rather poorly with rust-analyzer at the moment, making development cumbersome.

This crate contains:

• Parsing of derive inputs and sorbit's custom attributes
• An AST that can be parsed from syn's input
• An SSA IR that is lowered from the AST
• Generation of Rust code from the SSA IR

The crate is largely undocumented, and its API might change significantly.