Make S" work inside compiled words

The approach we're using here is kinda silly and cause by the fact that I
wasn't/still aren't very familiar with Forth outside of trying to implement it
myself.

From doing a bit of reading, it sounds like S" is primarily intended to be used
inside definitions -- not at runtime. So now we use the old, weird S" (that has a
single static buffer and only works at runtime) to bootstrap the system, then
replace it with a more sensible S" that only works at compile time.