Entry mappings of the form:
 "  02e00000-02e8a000: /foo/bin (deleted)"
are currently resulting in "(deleted)" being set as the build id.
Enforce the build id to be hex, and allow unrecognized junk to
be at the end of the mapping.

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