API: Add impl `From<*>` for `sem::TyKind` and correct some names of semantic types

[xFrednet]

This PR adds boilerplate code required for the implementation of https://github.com/rust-marker/marker/issues/141. I've also changed all semantic type construction, to use the TypedBuilder instead of the old new() function.

This PR also includes some changes, visible to API consumers, probably best summarized by the changelog entires:

• sem::TyKind now implements From<*> for all semantic types.
• Renamed sem::TyKind::FnTy -> sem::TyKind::Fn
• Renamed sem::TyKind::ClosureTy -> sem::TyKind::Closure

That's basically it. Some boring repetitive code changes, which will hopefully make it easier to expand semantic types in the future :D

[xFrednet]

The semver-check in our CI only found the two expected enum variant renames, I guess this should be good to go :D

Some to think about #141