Implement BREAK to exit REPT and FOR loops

Fixes #684

test/asm/break.asm

@@ -0,0 +1,22 @@
+FOR V, 1, 100
+      PRINTLN "- {d:V}"
+      IF V == 5
+          PRINTLN "stop"
+          BREAK
+      ENDC
+      PRINTLN "cont"
+ENDR
+WARN "done {d:V}"
+rept 1
+  break
+  ; skips invalid code
+  !@#$%
+elif: macro
+  invalid
+endr
+warn "OK"
+rept 1
+  if 1
+    break
+  no endc
+endr

test/asm/break.err

@@ -0,0 +1,5 @@
+warning: break.asm(9): [-Wuser]
+    done 5
+warning: break.asm(17): [-Wuser]
+    OK
+error: Unterminated IF construct (1 levels)!

test/asm/break.out

@@ -0,0 +1,10 @@
+- 1
+cont
+- 2
+cont
+- 3
+cont
+- 4
+cont
+- 5
+stop

test/asm/for.err

@@ -1,6 +1,6 @@
 ERROR: for.asm(16):
     FOR cannot have a step value of 0
-ERROR: for.asm(41) -> for.asm::REPT~5(47):
+ERROR: for.asm(41) -> for.asm::REPT~4(47):
     'v' already defined as constant at for.asm(41) -> for.asm::REPT~4(45)
-FATAL: for.asm(41) -> for.asm::REPT~5(47):
+FATAL: for.asm(41) -> for.asm::REPT~4(47):
     Failed to update FOR symbol value

test/asm/nested-break.asm

@@ -0,0 +1,17 @@
+n=1
+rept 10
+  print "A"
+  for x, 10
+    print "a"
+    if x==n
+      break
+    endc
+    print "z"
+  endr
+  if n==6
+    break
+  endc
+  println "Z"
+n=n+1
+endr
+println "\nn={d:n} x={d:x}"

test/asm/nested-break.out

@@ -0,0 +1,7 @@
+AazaZ
+AazazaZ
+AazazazaZ
+AazazazazaZ
+AazazazazazaZ
+Aazazazazazaza
+n=6 x=6