Add termTypePrec to TermLiteral to allow for TermLiteral SNat

[martijnbastiaan]

For #2270

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[DigitalBrains1]

I think this is going to mention a bunch of things you already fixed... because I started the review on an older version and after pressing "Refresh" it says I have 7 comments but I can only see two.

[DigitalBrains1]

I've deleted the outdated comments.

[leonschoorl]

Just like showsPrec has show, I think it makes sense to define a simpler entry function.
So when using it you can do something like:
showTermType (Proxy @a)
Instead of having to do:
termTypePrec 0 (Proxy @a) ""

Following the naming from "Show", it would make sense to then rename termTypePrec to showsTermTypePrec (or showsPrecTermType?).
I wouldn't put show equivalent in the class, like Show does, because I don't think it make sense to redefine it for individual instances. it can just be a standalone function.

[martijnbastiaan]

Done @leonschoorl

[DigitalBrains1]

I noticed some Haddock thingies.

[DigitalBrains1]

There's a convenience function for parentheses that makes the code short and sweet