magika: add ml-based content type detection

[hdonnay]

trafficstars

This is an experiment in using purego to do dynamic loading, to meet a $job requirement to sprinkle "AI" on things.

If anyone is interested in this seriously, there's additional work to be done on:

• Where can this get used (An rpm-like query function? An "anything of $type" function?)
• The integration tests need to pull in the runtime dependency.
• Double-checking the C API contract is being upheld.
• Double-checking the Go pointer-passing rules are being upheld.