Fix nested expansions being incorrectly handled

The biggest problem was simply that the length of children expansions was
not accounted for when skipping over the parent... this took a lot of
arduous debugging, but it finally works!

test/asm/equs-nest.asm

@@ -0,0 +1,4 @@
+X1 equs "Y1 equs \"\\\"Success!\\\\n\\\"\""
+Y1 equs "Z1"
+X1
+	PRINTT Z1

test/asm/equs-recursion.asm

@@ -1,2 +1,6 @@
-recurse EQUS "recurse"
-recurse
+recurse EQUS "recurse "
+recurse
+
+; FIXME: also handle the following:
+; recurse EQUS "recurse"
+; recurse