sol-compiler: 0.6 support

[dorothy-zbornak]

Description

Addresses #2528

This adds solidity 0.6 support to sol-compiler. I've also done some decent-sized refactors to (hopefully) make compatibility with new versions/features easier to integrate, and to tidy up the code. There are some other small changes listed below.

Parallelization

I rewrote a bunch of the code to be parallelized, to speed up build times.

Artifact Changes

• solc version string in artifact settings object is now normalized to {VERSION}+commit.{COMMIT_HASH}. Previously it could take on many forms, depending on where your compiler comes from (docker, npm, direct path).
• Paths for source files in the output artifacts are now expressed in dot-notation relative paths. Previously they were expressed as path segments, which made it difficult to tell if a source file was an npm module or not.
• Input artifacts now include the solidity version in their settings object. This makes automated verification a bit easier because we don't have to look up the solidity version used from the output artifacts as well.

Testing instructions

Types of changes

Checklist:

• Prefix PR title with [WIP] if necessary.
• Add tests to cover changes as needed.
• Update documentation as needed.
• Add new entries to the relevant CHANGELOG.jsons.

[jalextowle]

Looks pretty good! I have a few questions and comments, but it looks ready to merge once those are addressed.

[hysz]

LGTM! (~‾▿‾)~

[coveralls]

Coverage decreased (-0.05%) to 79.588% when pulling b9a68c0 on feat/sol-compiler/support-0.6 into 424cbd4 on development.