Rework methods on flexarray types

Put the pointer converting methods on the sized prefix types ([T; 0])
since they're the default type and what you're likely to be starting
with. Emphasize the ones which work on references since they have safe
lifetimes, but also have raw pointer variants, esp for handling
uninitialized cases.

The flex types have methods which return the sized type along with the
length.

Author: jsgf