Clarify Vector usage

[Nadrieril]

A Vector<I, T> is secretly an IndexVec<I, Option<T>>. There's something slightly misleading there: sometimes (e.g. in GenericParams) we expect all the values to be Some, sometimes (e.g. in the top-level fun_decls etc arrays) we intentionally allow for missing entries.

We should clarify the intent by splitting it into IndexVec and IndexMap, where the first one does not allow for vacant entries. When converting to OCaml we manually convert the map cases to a map anyway, so this will give us a more principled reason to do so.

[sonmarcho]

I 100% support this. ;)