Add "ROM"-numbered memory page to the total pool of memory for all devices

[ped7g]

Currently any DEVICE has only "RAM" type of memory pool for PAGE directive, but sometimes it may be handy to define symbols in "ROM" page for various symbol-export reasons (LABELSLIST, CSPECTMAP, SLD file).

I guess something like PAGE -1 (-2, -3, ... for devices where it makes sense to provide larger ROM space, like Next needs at least two 8kiB pages) can be added, PAGE 255 may clash with super-size devices (not sure if sjasmplus already has some with more than 256 pages, I think there is 4MB or 8MB "spectrum" device which has more than 256 pages).

Also consider extended EQU syntax for explicit mem-page definition for special cases when current mapping is not good enough... like:

MySymbol   EQU   $C000, 123    ; $$MySymbol == 123

(This is related to partially fixed #111)

[ped7g]

First try failed:
doing it like "-4 .. -1" pages just shifting internally everything by +4 doesn't fare well with absolute offsets/addresses allowed by savedev and savenex.

I guess the "ROMs" could rather use some hard constant like +0x7000 to pageNumber and +0x43210000 to memory offsets (limiting future device maximum RAM size to ~1GiB max by offset and ~400MiB by page number for 16kiB pages) ("strange numbers" to make them easy to remember) - and have completely separate detection of these and recalculating them into "virtual rom" memory.