assignable temporaries proposal with -fpermissive

[stsp]

This comment:
#56 (comment)
and the no_pnew branch (1db08dd)
represent the proposal of using assignable
temporaries under gcc with -fpermissive,
instead of placement new+(currently missing)
explicit lifetime management.

As gcc is finally supported, this can be
tried out. Needs to resolve the conflicts
with no_pnew branch first.

[stsp]

Whether or not this is interesting
if/when we switch to c++23 and use
explicit lifetime management, needs
to be evaluated. The aforementioned
comment states:

Even if the c++23
solution is applied, the alternatives will
remain still interesting, for example from
the gcc porting stand-point. They will then
migrate to https://github.com/dosemu2/fdpp/issues/225

But #225 is already closed...