Support ROM/RAM selection at compile-time

[tkers]

As described in https://www.taygeta.com/forth/dpanse.htm#E.5, we should add the words RAM and ROM to select the memory that words should be referencing.

Compile-time:

Word	Memory
,	ROM
c,	ROM
here	ROM/RAM
unused	ROM/RAM
allot	ROM/RAM
align	ROM/RAM
aligned	ROM/RAM
create	ROM/RAM
variable	ROM/RAM

Run-time:

Word	Memory
,	RAM
c,	RAM
here	RAM
unused	RAM
allot	RAM
align	RAM
aligned	RAM
create	unavailable
variable	unavailable

[davazp]

Why , and c, shouldn't be available for RAM as well? An early limitation that we can improve later or something intrinsic?

[tkers]

The most basic support would mean that RAM is uninitialised. So just like ! and c!, emitting data would only be supported for ROM (the other words only reference/allocate space).

I created #284 to discuss the possibility of initialising RAM.

[tkers]

Implemented in f270e7e

And updated to make VARIABLE possible in both locations: c9c5313