Revisited some generic concepts, callbacks, and some reorganization

- Callbacks for get/match, this does have a code cost, but allows more
  code reuse, which almost balances out the code cost, but also reduces
  maintenance and increased flexibility. Also callbacks may be able to
  be gc-ed in some cases.
- Consistent struct vs _t usage, _t for external-facing struct that
  shouldn't be messed with outside the library. structs for external and
  internal structs where anyone with access is allowed to modify.
- Reorganized several high-level function groups
- Inlined structures that didn't need separate definitions in header

Author: geky