ast::Type pretty printer 💅

[jmackie]

Stop using debug_render and write proper formatting logic.

Probably belongs in ditto-fmt (e.g. ditto_fmt::format_type).

There should be a heuristic for unnamed type variable formatting that attempts to assign them single letter names (a, b, c, ...) avoiding collisions with existing named variables.

Types should ideally be printed with consideration for their typing Environment - i.e. they should be rendered as they would appear in the module source given the imports.

Use it for type errors, the LSP #142, generated docs #63 etc.