ast: Refactor type checker and environment state

This commit does not change any functionality it just updates the
implementation so that type environment mutation only occurs within
the type checker. This makes it easier to reason about changes to the
type environment data structure(s). Specifically:

* The type environment constructor is now private. Callers do not need
  to instantiate type environments. This should only be done by the
  checker. No one appears to be using this constructor so while this
  is backwards incompatible, it should be safe.

* The schema set is now held by the checker as opposed to the type
  environment. The environment should not have to know anything about
  the schemas.

* The global input schema is now loaded by the compiler on init() on
  and provided as input to the checker along with other global options
  like the schema set. This avoids having the compiler reach into the
  type environment to perform updates.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>