Generate less bloaty code for errors::windows enums

[Swatinem]

Not really intended to be merged, feel free to ignore. This solves #847, but at the cost of tons of generated code.

This is just an experiment to see if custom Debug and from_u64/u32 code based on PHF leads to better codegen.
On my windows system, a full lto compile using cargo bloat shrinks the resulting binary of minidump-stackwalk by 100k, (from 10M to 9.9M), with a tiny regression to the debug data size.

The full story of what I did is documented here: https://swatinem.de/blog/optimizing-enums/

Ideally the Rust compiler would do this optimization internally and not generate such code in the first place.

[gabrielesvelto]

This is a nice improvement! I don't mind merging a bunch of generated code but would you prefer waiting to see if the compiler issue gets some traction first?