Implement LLVM-based Lexer for IR

[fivosts]

Adds 2 observations:

• Lexedir
• LexedirTuple

[fivosts]

@hughleat

[fivosts]

Not sure why 7 commits show up in the PR instead of just 2, probably because I branched from stable.

[codecov-commenter]

Codecov Report

Merging #742 (48c0ec5) into development (b58e83b) will decrease coverage by 0.02%. The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #742      +/-   ##
===============================================
- Coverage        88.75%   88.73%   -0.03%     
===============================================
  Files              129      130       +1     
  Lines             7855     7864       +9     
===============================================
+ Hits              6972     6978       +6     
- Misses             883      886       +3     

Impacted Files	Coverage Δ
compiler_gym/envs/llvm/lexed_ir.py	100.00% <100.00%> (ø)
compiler_gym/envs/llvm/llvm_env.py	91.66% <100.00%> (+0.04%)	:arrow_up:
compiler_gym/envs/llvm/specs.py	100.00% <100.00%> (ø)
compiler_gym/service/service_cache.py	89.47% <0.00%> (-2.64%)	:arrow_down:
compiler_gym/service/connection.py	75.91% <0.00%> (-1.68%)	:arrow_down:
...loop_tool/service/loop_tool_compilation_session.py	89.86% <0.00%> (+2.02%)	:arrow_up:

[ChrisCummins]

Note the "Pre-commit" job is failing with a few BUILD file errors. You could run buildifier locally or just copy the patch from the CI output

[fivosts]

Rebasing to dev probably did this. Did fresh rebase, removed broken symlinks, ran buildifier and cherry picked my commits, should be fine now. Let me know if there's anything else I have missed

[ChrisCummins]

LGTM. The CI test failures are known problems and not caused by this change. Merging, thanks @fivosts!

Cheers, Chris