Clean up indexing, Pt. 1

Closes #46 (closed).

• Remove dead and duplicate code
• Introduce type alias for index
• Fix typing issues
• Bump mypy to 1.15.0 (current latest version). "Narrowing Tuple Types Using len()" was introduced in 1.7, we've been using 1.2.

#41 remains to be resolved later, after !39 lands.