Add versioning to the binary profile format

[michaelwoerister]

Right now the files generated by rustc -Zself-profile don't contain any indication of what their concrete encoding is. That should change so that post-processing tools support more than one version or at least give a sensible error message when encountering an unsupported encoding.

For example, each file could start with a some file magic (b"MMES" for the event stream, b"MMSD" for the string table data, and b"MMSI" for the string table index) and a 4 byte little-endian version number that can be used to select the right decoder.