New structured macro: for loops

This macro assumes a register increment.

Some examples:
_for(a,#0)
_for_end(a,#20)

    ;from 10 to 20
    ld      e,#20
    _for(a,#10)
    _for_end(a,e)

    ;16 bit counter
    _for(bc,#0)
    _for_end(bc,#0x23FF)

    _for(de,#0x2000)
    _for_end(de,hl)