Support code coverage using llvm-gcov (fix #6)

[mssun]

• use gcov (llvm-gcov)
• CI can work properly

[mssun]

continuous-integration/drone/pr — the build failed

This should be fixed in CI. By default, the CI will not provide secret to for PR event.

[Arcterus]

This seems at least better than what we have right now as it does work, but I'm not so sure about the accuracy. On Coveralls, it noted that src/head had 70.2% coverage, but then the only file in src/head is src/head/mod.rs, which supposedly has 38.1% coverage? If you look at the reported coverage for the source code, it always (or at least almost always) differs from the coverage for the directory.

I think technically the reported coverage for the source code itself is more accurate, as it's mentioning missed branches for parsing failures and such, but I'm not sure why the directory coverage differs?

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@ea2628e). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #7   +/-   ##
=========================================
  Coverage          ?   27.34%           
=========================================
  Files             ?       23           
  Lines             ?     2370           
  Branches          ?      519           
=========================================
  Hits              ?      648           
  Misses            ?     1572           
  Partials          ?      150

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