Store the nested expansions starting from the deepest one (#829)

This shortens the lexer by 100 lines and simplifies
access to expansion contents, since it usually needs the
deepest one, not the top-level one.

Fixes #813

test/asm/nested-expansions.asm

@@ -0,0 +1,6 @@
+def foo equs "bar"
+def bar equs "qux"
+MACRO test
+	\1
+ENDM
+	test foo 0

test/asm/nested-expansions.err

@@ -0,0 +1,3 @@
+ERROR: nested-expansions.asm(6) -> nested-expansions.asm::test(4):
+    Macro "qux" not defined
+error: Assembly aborted (1 error)!