This finishes the removal of ConstString from FileSpec's interface. Note
that FileSpec is still _backed_ by ConstStrings, that will be changed in
a follow-up.
A non-goal for this change is to rewrite business logic, so I kept the
type of some variables as ConstString where a refactor would necessary
to change that. I converted printf-style formatting to formatv-style
formatting where required, and otherwise tried to leave callsites alone
to the extent that was possible.
Alex Langford
f9b5264523