add `PROGENITOR_DEBUG=<PATH>` environment variable to output macros to files

[ahl]

Sometimes users want to see the code the generate_api! macro produces. cargo-expand works, but it expands a lot of stuff. I think it would be useful to have an environment variable one could set that would result in the macro output landing in some file to examine after the build.

[jclulow]

Could it just always emit the files, into some place under target/? Or, at least, always emit them if some option is passed to generate_api!()?

[ahl]

It could always mine bitcoin for 100ms while no one was looking! it's just software!

[andrewjstone]

+1 for just always writing them out, and possibly issuing a build log about where to find them to the console.

[ahl]

I think build times may be slow enough already, but I'll think about it

[drahnr]

I faced the same issue in #418 ( https://github.com/oxidecomputer/progenitor/pull/418/files#diff-249400e9b67720670451815acee092c499e96d31a9beefcc5cc63a6443d8bc5aR285-R296 in particular ) using https://github.com/drahnr/expander to expand a single TokenStream to file for the sake of debugging.