SOAS package and better TH support for complex types

[fizruk]

This PR builds on top of #27.

Here SOAS package serves merely as an example of a more complex grammar/types to generate Free Foil for. Later, it should also have proper library definitions.

This PR is a reimplementation of TH generation for Free Foil, providing simultaneous definition for many types:

• Signatures (e.g. terms and parts of terms)
• Pattern synonyms
• Scope wrappers
• Bindings (e.g. patterns)
• Quantifiers (e.g. metavar substitutions, constraints)
• Conversion functions

What is NOT supported (and is not planned in this PR):

• separate scope namespaces (e.g. if you put a type annotation in a term (or term pattern), it will be "dependent" in the sense that its scope will be shared/related to the term's scope); supporting separate namespaces would require more substantial change to the AST type, e.g. something like hypertypes

• CoSinkable instances are not generated here, instead I plan to implement a generic version based on kind-generics, similarly to ZipMatchK

Note that this PR contains 1k+ LoC of Template Haskell, which means there are probably some bugs 🐞