Add Package.swift

[milch]

Hey there,

Not sure if your fork will accept PRs, but wanted to submit it anyway since it seems to be more recently maintained compared to upstream.

This PR adds SPM support. SPM unfortunately is very peculiar about where files need to be placed:

• It does not support RocksDB's style of placing test files next to source files very well, hence the long list of source files and excludes
• You can only define a single folder for public headers, which is why I had to move those into a separate include dir
• I found no way to generate files during the build with SPM, so there's some logic there to replace the generated file with defines during the build