Improve code organization and build process

[jaredlundell]

I'm interested in using FSST, but the current setup of the code makes it a bit difficult to adopt (similar to the discussion in #34). There are a few changes I'd be interested in making to simplify adoption:

• Build shared objects in addition to statically linked binaries to enable dynamic linking
• Reorganize the directory structure to separate code from documentation and test data
• Add unit tests
• Allow customization of the march compiler option instead of always using march=native, since build machines don't always have all the same processor features as deployment targets.

If you're open to these types of contributions, I'm happy to make the changes and send PRs.