refactor(developer): kmc - encapsulate strs and elem items for sort

Fixes #7235.

Encapsulates all strings in the in-memory compile so that we can binary
sort them for build. This means that finalization of strs and elem is
no longer needed -- it's part of setup of build_strs and build_elem --
so that is factored away. Renamed alloc_string and alloc_element_string.

It may be worth renaming some of the ELEM_STRING objects / interfaces as
currently they are a little confusing.