std::vec: Remove the function same_length

The basic construct x.len() == y.len() is just as simple.

This function used to be a precondition (not sure about the
terminology), so it had to be a function. This is not relevant any more.
std::at_vec and vec: Unify build_sized, build_sized_opt into build

These functions have very few users since they are mostly replaced by
iterator-based constructions.

Convert a few remaining users in-tree, and reduce the number of
functions by basically renaming build_sized_opt to build, and removing
the other two. This for both the vec and the at_vec versions.