Implement FOREACH (#658)

This acts like `REPT` with a variable automatically
incremented across a range of values

Fixes #432

test/asm/foreach.asm

@@ -0,0 +1,48 @@
+foreach n, 10
+	printt "{d:n} "
+endr
+	printt "-> {d:n}\n"
+
+foreach v, 0
+	printt "unreached"
+endr
+
+foreach v, 2, 1
+	printt "unreached"
+endr
+
+foreach v, 1, 2, 0
+	printt "unreached"
+endr
+
+foreach x, 1, 5+1
+	printt "{d:x} "
+endr
+	printt "-> {d:x}\n"
+
+foreach v, 10, -1, -1
+	printt "{d:v} "
+v = 42
+endr
+	printt "-> {d:v}\n"
+
+foreach q, 5, 21, 5
+	printt "{d:q} "
+purge q
+endr
+	printt "-> {d:q}\n"
+
+s EQUS "x"
+foreach s, 3, 30, 3
+	printt "{d:x} "
+endr
+	printt "-> {d:x}\n"
+
+foreach v, 10
+	printt "{d:v}\n"
+if v == 3
+purge v
+v equ 42 ; causes a fatal error
+endc
+endr
+	printt "-> {d:v}\n"

test/asm/foreach.err

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

test/asm/foreach.out

@@ -0,0 +1,9 @@
+0 1 2 3 4 5 6 7 8 9 -> 10
+1 2 3 4 5 -> 6
+10 9 8 7 6 5 4 3 2 1 0 -> -1
+5 10 15 20 -> 25
+3 6 9 12 15 18 21 24 27 -> 30
+0
+1
+2
+3