Generate gcov coverage data

[brson]

It would be nice to know what our test coverage looks like. I believe this depends on #689.

[msullivan]

Depends on #237.

[kud1ing]

Should this issue be tagged with "debuginfo"?

[brson]

Yes

[eholk]

According to a comment in #2234, this bug is unblocked now.

[brson]

This is definitely blocked again on functioning debug info.

[bblum]

revisiting for triage. this would be especially great to have in the concurrency tests, where covering all the branches is a function of scheduling rather than input values.

[bblum]

i was already here, but now i am here again

[jdm]

This would be super-interesting to dig into now, since we have quite functional debug symbols after this summer's GSOC.

[catamorphism]

Would be awesome, but not 1.0

[mletterle]

On a related note, I found this project today: https://github.com/SimonKagstrom/kcov

kcov uses the DWARF debug-info to generate coverage data, I've verified that it appears to work with test crates. It also generates cobertura xml files of the coverage data. Might be a nice stop gap for getting coverage numbers on rust itself.

[emberian]

kcov looks much nicer than gcov overall.

[pongad]

I'm not sure how relevant this is, but Go did it in a pretty interesting way: http://blog.golang.org/cover
Not sure if everyone wants to do it the way they did.