Add CI code coverage

[willglynn]

This changeset changes the Travis CI config to run tests as usual, but assuming cargo test passes, it uses after_success: to download and build kcov, re-runs the test executable to collect code coverage info, and posts it to codecov.io.

Here's Travis build output and the codecov coverage report from this branch on my repo if you'd like to see what it looks like. All the after_success: stuff is all collapsed on Travis by default, so it's nowhere near as noisy as you might think based on the .travis.yml.

You would need to sign up with Codecov before merging, though like Travis, it's free for open source software.

Finally, it's worth noting that while the code coverage data is correct on a line by line basis, the code coverage statistics are incorrect (rust-lang/rust#39293). The coverage information shows which code paths get executed and which do not, but generic functions which were never codegen'd for the test executable do not count against the coverage percentage. For example, RudyMap shows 100% coverage… but the report also indicates that remove(), contains_key(), get_mut(), len(), and is_empty() are never called.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2374879). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #21   +/-   ##
=========================================
  Coverage          ?   58.87%           
=========================================
  Files             ?       17           
  Lines             ?     1082           
  Branches          ?        0           
=========================================
  Hits              ?      637           
  Misses            ?      445           
  Partials          ?        0

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[willglynn]

Oh. I guess you don't need to sign up, or maybe you already did.

[willglynn]

I added some tests which exercise the RudyMap public interface and cause most other things to get generated. Coverage stats are now better, though the underlying issue remains.