[service] Add a CompilationSession abstract base class.

[ChrisCummins]

The CompilationSession class encapsulates an incremental compilation
session.

This is the first in a series of patches that separates the RPC server
from the compilation session.

Issue #254.

[codecov-commenter]

Codecov Report

Merging #261 (f6b63e1) into development (5d20b14) will decrease coverage by 0.20%.
The diff coverage is n/a.

❗ Current head f6b63e1 differs from pull request most recent head b54a667. Consider uploading reports for the commit b54a667 to get more accurate results

@@               Coverage Diff               @@
##           development     #261      +/-   ##
===============================================
- Coverage        83.33%   83.12%   -0.21%     
===============================================
  Files               76       76              
  Lines             4338     4338              
===============================================
- Hits              3615     3606       -9     
- Misses             723      732       +9     

Impacted Files	Coverage Δ
compiler_gym/envs/compiler_env.py	86.33% <ø> (-0.59%)	⬇️
compiler_gym/service/proto/__init__.py	100.00% <ø> (ø)
compiler_gym/service/connection.py	74.16% <0.00%> (-2.59%)	⬇️

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[ChrisCummins]

Marking this ready for review but no rush to merge it while we're still discussing the design in #254.

[hughleat]

If the protobuf stuff is okay to keep then this LGTM :-)

[hughleat]

Just curious: why doesn't this return a status like the c++ code?

[ChrisCummins]

We don't use exceptions in C++ (FWIW I don't have particularly strong opinions on the matter), the CompilationSession constructor in C++ is a safe default, and any of the three methods that can fail (init, compute feature, run action) return Status.

For Python, exceptions are fine and all of the runtime logic that interacts with CompilationSession will catch exceptions and set the relevant RPC error code