Reorganize module hierarchy

[kozross]

This is the first attempt to try and make some sense of the module hierarchy. The overall goals were:

1. Users should need to only import Plutarch.Prelude and get everything they need
2. Plutarch itself shouldn't import Plutarch.Prelude, only things in the Plutarch.Internal.* namespace
3. plutarch-ledger-api and plutarch-testlib should rely minimally on imports from Plutarch.Internal.*

Additionally, there were a couple of changes:

1. All builtins should have a pbuiltinMyBuiltinName in Plutarch.Internal.Builtin, allowing them to be called without having to wonder what their signatures are.
2. Plutarch.* has been significantly emptied.
3. Plutarch.Internal.Builtin contains (most of) the types that we have to have because they're mandated by Plutus.
4. Some modules got merged.

This is not the final outcome, I just wanted to get something reviewed and merged, or the diffs would get too huge.

[SeungheonOh]

This is quite massive. More so than I thought, would it be better to make Plutarch.Builtins.XYZ modules instead?

[kozross]

Yeah, I agree.