Refactor of file path functionality

[kbaum]

• File hashing converts to absolute path before hashing
• Both relative and absolute file conversion cache results
• Moved file path helper functionality to classes to make more testable and separate concerns

Memory Impact

Before:

bundle exec rake benchmarks:run_big
D, [2019-08-04T21:09:07.973428 #51553] DEBUG -- : using default configuration
Total allocated: 113157075 bytes (821933 objects)
Total retained:  666256 bytes (4015 objects)

allocated memory by gem
-----------------------------------
  49874880  coverband/lib
  30916130  json
  21175024  redis-4.1.2
  11191041  other

After

D, [2019-08-04T21:13:09.517425 #52381] DEBUG -- : using default configuration
Total allocated: 109734929 bytes (821922 objects)
Total retained:  666256 bytes (4015 objects)

allocated memory by gem
-----------------------------------
  49874880  coverband/lib
  30916130  json
  19585080  redis-4.1.2
   9358839  other

[kbaum]

This has been merged in as part of another PR.