z80: implement stack overflow error condition

Also, remove underflow checks from next: it's taxing and useless
because underflow check already takes place in each relevant native
word.

blk/076

@@ -1,6 +1,11 @@
-STACK OVERFLOW PROTECTION: To avoid having to check for stack
-underflow after each pop operation (which can end up being
-prohibitive in terms of costs), PS_ADDR should be set to
-at least 6 bytes before its actual limit. 6 bytes because we
-seldom have words requiring more than 3 items from the stack.
-Then, at each "exit" call we check for stack underflow.
+Stack underflow and overflow: In each native word involving
+PSP popping, we check whether the stack is big enough. If it's
+not we go in "uflw" (underflow) error condition, then abort.
+
+We don't check RSP for underflow because the cost of the check
+is significant and its usefulness is dubious: if RSP isn't
+tightly in control, we're screwed anyways, and that, well
+before we reach underflow.
+
+Overflow condition happen when RSP and PSP meet somewhere in
+the middle. That check is made at each "next" call.

blk/284

@@ -1,5 +1,4 @@
 PC ORG @ 1 + ! ( main )
-( STACK OVERFLOW PROTECTION: See B76 )
     SP PS_ADDR LDdn, IX RS_ADDR LDdn,
 ( LATEST is a label to the latest entry of the dict. It is
   written at offset 0x08 by the process or person building

blk/286

@@ -2,14 +2,14 @@ lblnext BSET PC ORG @ 0x1b + ! ( next )
 ( This routine is jumped to at the end of every word. In it,
   we jump to current IP, but we also take care of increasing
   it by 2 before jumping. )
-	( Before we continue: are stacks within bounds? )
-    ( PS ) HL PS_ADDR LDdn,
-    SP SUBHLd,
-    JRC, lblofl BWR ( abortUnderflow-B285 )
-    ( RS ) IX PUSH, HL POP,
-    DE RS_ADDR LDdn,
-    DE SUBHLd,
-    JRC, lblofl BWR ( IX < RS_ADDR? abortUnderflow-B285 )
+	( Before we continue: are we overflowing? )
+    IX PUSH, EX(SP)HL, ( do EX to count the IX push in SP )
+    SP SUBHLd, HL POP,
+    IFNC, ( SP <= IX? overflow )
+        SP PS_ADDR LDdn, IX RS_ADDR LDdn,
+        DE BIN( @ 0x13 ( oflw ) + LDdd(n),
+        JR, L3 FWR ( execute, B287 )
+    THEN,
     LDA(BC), E A LDrr, BC INCd,
     LDA(BC), D A LDrr, BC INCd,
     ( continue to execute )

blk/287

@@ -1,4 +1,4 @@
-lblexec BSET L1 FSET ( B284 ) L2 FSET ( B285 )
+lblexec BSET L1 FSET ( B284 ) L2 FSET ( B285 ) L3 FSET ( B286 )
     ( DE -> wordref )
     ( When we have a BIN( offset, we need to adjust stable
       ABI offsets. )