[llvm] Add a new Bitcode observation space.

[ChrisCummins]

This adds a Bitcode observation space that returns the bitcode of an
LLVM module directly as a byte array, without having to go via an
intermediate file.

Fixes #426.

[codecov-commenter]

Codecov Report

Merging #442 (b2dc549) into development (4ba9a7a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #442      +/-   ##
===============================================
+ Coverage        87.36%   87.38%   +0.02%     
===============================================
  Files              104      104              
  Lines             5966     5969       +3     
===============================================
+ Hits              5212     5216       +4     
+ Misses             754      753       -1     

Impacted Files	Coverage Δ
compiler_gym/spaces/sequence.py	96.96% <100.00%> (+0.30%)	⬆️
compiler_gym/views/observation.py	96.87% <0.00%> (-3.13%)	⬇️
compiler_gym/views/observation_space_spec.py	90.42% <0.00%> (-1.07%)	⬇️
compiler_gym/datasets/dataset.py	85.71% <0.00%> (-0.80%)	⬇️
compiler_gym/envs/llvm/datasets/cbench.py	78.37% <0.00%> (-0.78%)	⬇️
compiler_gym/service/connection.py	77.58% <0.00%> (+1.03%)	⬆️
compiler_gym/envs/llvm/llvm_benchmark.py	89.11% <0.00%> (+2.04%)	⬆️

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[hughleat]

Perfect :)