External matchfinder API

[embg]

This is a code-complete draft PR for the external matchfinder API. It is ready for code review, but still needs some documentation work and build tweaks.

Here is a list of TODOs to exit draft status:

• [x] Fix copyright headers
• [x] Fix cmake build for zstreamtest
• [x] Add externalMatchfinder demo program to make contrib
• [ ] Add README.md for externalMatchfinder demo
• [ ] Write zstd.h documentation for the API
• [ ] Update PR summary
• [ ] Fix @Cyan4973 code style nits
• [ ] Document constraint that matchLen != 0 for all but the last sequence. Test that condition (if in a debug build) in postProcessSequences.
• [ ] Add windowSize, verify it in debug builds

I'm opening the PR now to get early feedback on the code itself. The remaining TODOs are orthogonal and can be reviewed after the code itself is finalized.