Moved proto-compiler under tools and fixed main.rs paths

[greg-szabo]

Closes #755 .
Part of #723 .

• Moved the proto-compiler folder.

• In main.rs, relative paths were updated to accommodate the new path.

• In main.rs OS-specific folder separators were accommodated. (instead of /, we're using PathBuf::join)

• Referenced an issue explaining the need for the change

• Updated all relevant documentation in docs

• Updated all code comments where relevant

• Wrote tests

• Updated CHANGELOG.md

[codecov-io]

Codecov Report

Merging #754 (a9e313e) into master (8bbb09b) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #754   +/-   ##
======================================
  Coverage    43.1%   43.1%           
======================================
  Files         204     204           
  Lines       13620   13620           
  Branches     3571    3571           
======================================
  Hits         5874    5874           
  Misses       7369    7369           
  Partials      377     377           

Impacted Files	Coverage Δ
tools/proto-compiler/src/constants.rs	0.0% <ø> (ø)

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