…ains it

To improve startup performance, we will later load smaller `SyntaxSet`s instead
of one giant one. However, the current API assumes only one `SyntaxSet` is ever used,
and that that implicitly is the `SyntaxSet` from which returned `SyntaxReference`s
comes.

This change changes the API to reflect that `SyntaxSet` and `SyntaxReference`
are tightly coupled, and enables the use of several `SyntaxSet`.