backtrace: highlight frames that point to local code

[japaric]

using the heuristic: if the file location of the function is relative to the current directory then
it's "local code"

before:

after:

I don't have too many opinions about how to highlight these frames so treat the above as a strawman proposal

cc #139 @Lotterleben

[jonas-schievink]

As a followup, we might be able to check if the path is part of the containing Cargo workspace instead, that might improve the accuracy a bit.

bors r+

[bors]

Build succeeded:

• test (nightly, macOS-latest)
• test (nightly, ubuntu-latest)
• test (nightly, windows-latest)
• test (stable, macOS-latest)
• test (stable, ubuntu-latest)
• test (stable, windows-latest)

[Lotterleben]

This is brilliant!