z80a: de-variable-ize

Use straight VARIABLE instead of Z80MEM+. Initially, I used this
system to allow z80a to be embedded in a system binary, but now
I don't think it's worth it. Compiled, z80a is 2.5k. Sure, it's a
sizeable amount of RAM, but I think that even with it in RAM, I'll
manage a bootstrap within my most constrained machine, the SMS with
8K.

blk/081

@@ -5,7 +5,7 @@ RAMSTART  FUTURE USES          +3c       BLK(*
 +08       C<* override         +53       (emit) override
 +0a       NLPTR                +55       (key) override
 +0c       C<*                  +57       FUTURE USES
-+0e       WORDBUF              +5b       z80a's variables
++0e       WORDBUF              
 +2e       BOOT C< PTR          +5d       adev's variables
 +30       IN>                  +5f       FUTURE USES
 +32       IN(*                 +70       DRIVERS

blk/212

@@ -1,2 +1 @@
-1 LOAD+ Z80A$
-3 37 LOADR+
+1 37 LOADR+

blk/213

@@ -1,9 +1,6 @@
-: Z80AMEM+ 0x5b RAM+ @ + ;
-: ORG 0 Z80AMEM+ ;
-: BIN( 2 Z80AMEM+ ;
-: L1 4 Z80AMEM+ ; : L2 6 Z80AMEM+ ;
-: L3 8 Z80AMEM+ ; : L4 10 Z80AMEM+ ;
-: Z80A$ H@ 0x5b RAM+ ! 12 ALLOT 0 BIN( ! ;
+VARIABLE ORG
+VARIABLE BIN( 0 BIN( !
+VARIABLE L1 VARIABLE L2 VARIABLE L3 VARIABLE L4
 : A 7 ; : B 0 ; : C 1 ; : D 2 ;
 : E 3 ; : H 4 ; : L 5 ; : (HL) 6 ;
 : BC 0 ; : DE 1 ; : HL 2 ; : AF 3 ; : SP AF ;