Refactor procedural macros

[rory-ocl]

The procedural macro implementations have become a bit unwieldy to maintain and add features to. They are essentially one function each, which has continued to grow in size as more features are implemented. The macros should be refactored into smaller pieces that can be more easily tested and documented.

[rory-ocl]

Once extract_if is stable we may be able to use that to simplify attribute parsing.
rust-lang/rust#43244