Moving from Rust nightly to stable

[AlexMikhalev]

Required Functionality I have to pin the Rust toolchain to nightly-2022-08-01 in many places as it's the last one when synth compiles.

Proposed Solution Move to Rust stable

**Current unstable features used in Synth***

/dist/playground/src/main.rs
1:#![feature(try_blocks)]
./gen/src/lib.rs
1://#![feature(try_trait)]
./core/src/lib.rs
1:#![feature(async_closure, map_first_last, box_patterns, error_iter, try_blocks)]
./synth/src/lib.rs
1:#![feature(async_closure, map_first_last, box_patterns, concat_idents, error_iter)]

we need to build a roadmap - a path to stabilization, technical debt which needs to be paid off.

[iamwacko]

I have been looking into this. It looks like what we need to do is:

• [x] Remove #![feature(async_closure)], as I don't see the feature being used anywhere. At the very least it definitely isn't in core.
• [ ] Find some way other way to find the final error, so we won't need error_iter. Or just change the way errors are reported entirely.
• [x] Find some alternative to map_first_last, maybe a crate, as BTreeMap's are used frequently enough to make this hard to remove.
• [ ] Find a replacement for box_patterns. box_patterns allows the use of match on boxes, and Synth has a lot of boxes.
• [ ] try_blocks could be removed if we just move all of the contents of the blocks to separate functions, but I don't see many cases of it.
• [ ] concat_idents can be removed, as I don't see it used anywhere in the code.
• [ ] Figure out what try_traits is and how to remove it. I couldn't find this in the unstable book, so I don't know how to deal with it.

[iamwacko]

map_first_last was made stable in the latest release.