As far as I know, only a few users actually use it, but these changes
allow us to avoid including a surprising amount of code in the main
compilation path.
In the future we can expect many more matchers, so let's give them
a place to live.
Also moved matcher-related internal files to `internal` subfolder.
Ideally we should sort out all of our source code, but that will
have to come later.
Now that the recommended distribution and usage method is proper
library, users can just avoid including the matcher headers to get
basically the same effect.
