+ No functional change here, just renames and #include changes.
+ CMake can't have slashes in the target names. I'm suspciious of
that weird machinery for test, but I made it work.
+ A couple of builtins did not include their own headers, that
is no longer the case.
Prior to this change, builtins would take their arguments as `wchar_t **`.
This implies that the order of the arguments may be changed (which is
true, `wgetopter` does so) but also that the strings themselves may be
changed, which no builtin should do.
Switch them all to take `const wchar_t **` instead: now the arguments may
be rearranged but their contents may no longer be modified.
I did not realize builtins could safely call into the parser and inject
jobs during execution. This is much cleaner than hacking around the
required shape of a plain_statement.
