feat: asynchronous code generation

[Kha]

This PR enables code generation to proceed in parallel to further elaboration.

It does not aim to make further refinements such as generating code for different declarations in parallel or removing the dependency on kernel checking.

[Kha]

!bench

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 757899a7d179b3fd845d3e03031bbe360c80ef51 --onto c70f4064b4041dee70925bddbef095ebb2b36d7d. (2025-01-24 19:04:29)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 757899a7d179b3fd845d3e03031bbe360c80ef51 --onto 62788395345c7f145714157c138acd0e137ab716. (2025-01-26 20:08:50)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 757899a7d179b3fd845d3e03031bbe360c80ef51 --onto 20c616503abe5ce4253c56dbcd7766a91c675ba0. (2025-01-29 13:08:18)

[leanprover-bot]

Here are the benchmark results for commit 63ac64b.
There were significant changes against commit 757899a:

  Benchmark                   Metric                  Change
  ====================================================================
- Init.Prelude async          branches                  2.3% (275.1 σ)
- Init.Prelude async          instructions              1.8% (253.4 σ)
- bv_decide_inequality.lean   task-clock                1.9%  (10.2 σ)
- ilean roundtrip             parse                     3.0%  (24.3 σ)
- parser                      task-clock                4.0%  (17.9 σ)
- parser                      wall-clock                4.0%  (18.3 σ)
- stdlib                      attribute application     1.9%  (18.2 σ)
- stdlib                      type checking             2.0% (210.4 σ)

[Kha]

@zwarich FYI, you might want to take a look even though this interacts with the code generator only superficially, in compileDecls. The unification of that and compileDecl is desirable, I hope :) .