add --shorten-paths

[japaric]

this flags compresses the paths to crates.io dependencies from full paths to the format [$cratename-$crateversion]/src/module/file.rs

cc #66

---

before:

$ cargo rb hello
(..)
stack backtrace:
(..)
   4: main
        at src/bin/hello.rs:6:1
   5: ResetTrampoline
        at /home/japaric/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rt-0.6.13/src/lib.rs:547:26
   6: Reset
        at /home/japaric/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rt-0.6.13/src/lib.rs:550:13

after:

$ cargo rb hello
(..)
stack backtrace:
(..)
   4: main
        at src/bin/hello.rs:6:1
   5: ResetTrampoline
        at [cortex-m-rt-0.6.13]/src/lib.rs:547:26
   6: Reset
        at [cortex-m-rt-0.6.13]/src/lib.rs:550:13

currently only paths in the backtrace are modify (because that was the easiest thing to implement / test)

TODO:

• [x] also compress paths in defmt logs

[jonas-schievink]

We might want to give this option a shorter name, something like --shorten-paths maybe. This option could then be extended to cover paths to the standard library without having to add another one.

[japaric]

bors r=jonas-schievink

[bors[bot]]

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)