Language: replace indices with indexes

[addisoncrump]

We should unify our use of "indices" and "indexes"; presently, we use these effectively interchangeably around the codebase, but this has led to some inconsistent naming of types/information in documentation. Similarly, any types/functions we rename that existed in the previous release should have a second copy with the original name (e.g. a pub type for types, pub fn that calls the intended function for functions) and a #[deprecated] tag explaining the change.

This should preferably be completed after #1886 is merged.

[addisoncrump]

cc @domenukk @andreafioraldi @tokatoka: do we prefer "indices" or "indexes"? They are both valid; "indices" is primarily used in mathematical contexts, "indexes" for other fields.

See conversation below; we want to unify on "indexes".

[tokatoka]

for me indexes

[domenukk]

Looks like rust string uses char_indices - but otherwise we have way more uses of indexes in the codebase.

[addisoncrump]

Okay, then indexes should be used.

[domenukk]

This is done I think