fix: realising of built-in types, hard-coded aliases

[gerau]

Closes #235 and partially closes #202.

There are 3 small and separate fixes:

1. Remove hard-coded built-in aliases from the parser, so all alias checking is in types.rs
2. Adds check for redefining built-in aliases and types in TypeAlias parsing function. It uses the same logic as the first fix, so I included it here.
3. Fix small typo within TypeInner display function -- there are missing whitespace after comma inside Either type. You can see this issue in this PR for simplicitiy-lang-org. It's a small enough change that I thought I could include it here.

Would be happy to split this up if needed.