Move common types to a runtime lib, implement location tracking

[osa1]

This moves code common to all lexers to a runtime library called lexgen_util. This library is not to be used by users.

This has a few advantages:

• Code generator becomes simpler and generates less code

• It becomes possible to define multiple lexers in the same scope

• We avoid generating same types again for things like locations in the input. In addition to redundancy, this causes problems with nominal typing (i.e. Location types for different lexers become distinct).

This also implement location tracking. Instead of byte indices, we now return

struct Loc {
    line: u32,
    col: u32,
    byte_idx: usize,
}

Fixes #26

[osa1]

This is ready, just need to update README and CHANGELOG.