WIP: Improve crate compilation times

[Michael-F-Bryan]

Hopefully this will make improvements towards #1017. As I'm going I'll try to post numbers so we can see how changes affect compilation times.

Initial breakdown of compilation times using cargo build -Ztimings:
timings.zip

[Michael-F-Bryan]

After updating Cargo.lock with cargo update and manually upgrading env_logger from 0.6 to 0.7, compilation time dropped from 97.7s to 85.9s.

I'm guessing the compilation improvements are because we aren't compiling multiple versions of the same crate.

bump-dependency-versions.zip

[ehuss]

Thanks! Looks like it bumps msrv to 1.37 (I think)? Would be fine with me.

[Michael-F-Bryan]

For some reason, removing the itertools crate (it was only used two or three times to skip an allocation when concatenating lines) and running cargo clean && cargo build -Ztimings again dropped compilation times on my machine to 60s.

remove-itertools.zip

I don't think itertools was to blame... maybe it had something to do with build order or a warm page cache?

New dependency graph:

[Dylan-DPC-zz]

@ehuss yeh most likely this will be a breaking change so might as well release a new major version when finished which would also justify the msvr bump

[Michael-F-Bryan]

I just removed toml-query and the "build" stage in CI went from 4 mins 11 secs to 3 mins 49 secs.

New dependency graph:

Timings: remove-toml-query.zip

[Michael-F-Bryan]

I swapped out iron with simple-server and the "build" stage in CI went from 4 mins 1 second to 3 mins 52 seconds.

Timings:
remove-iron.zip

[ehuss]

@Michael-F-Bryan Is this ready to go? You still have WIP in the title. I really like the changes, and it looks good to go to me.

[Michael-F-Bryan]

I'm not certain it makes much of a difference. When running on my laptop I found the results super noisy and inconsistent, and I wasn't confident the compile time improvements weren't just due to something like the page cache or CPU governor changing clock frequency...

[ehuss]

Thanks for getting this going! Closed via #1211.