Simplify regexps and define a recommended format:
  Start   End     object file name     offset(optional)   linker build id
  0x40000-0x80000 /path/to/binary      (@FF00)            abc123456

Also include some additional tests and move existing tests to legacy_profile_test.go,
closer to the code in legacy_profile.go.

Also update the build instructions to use go get