libstdc++/GCC compatibility

Only functional change is adding a fast path to str_prefix::to_uint.
Also addressed certain warnings mostly pertaining to switches without
default cases.

I don't actually understand why the fda.h falldown_array::Funs change
was necessary, but as it's only used in a static_assert the change
shouldn't be able to affect anything and strictly speaking this is less
code.

New bug: coroutine use-after-free in basic_parser.