Prohibit nested macros

After discussion (starting there:
https://github.com/gbdev/rgbds/pull/594#issuecomment-706437458
), it was decided that plain nested macros should not be
allowed.
Since #590 is fixed, EQUS can be used as a workaround;
multiline strings (#589) will make that easier on the
user when implemented.
Fixes #588, supersedes and closes #594.
Additionally, closes #388.

test/asm/nested-macrodef.asm

@@ -0,0 +1,27 @@
+outer_ok: MACRO
+definition equs "inner_ok: MACRO\nPRINTT \"Hello!\\n\"\nENDM"
+definition
+	PURGE definition
+ENDM
+
+	outer_ok
+	inner_ok
+
+
+outer_arg: MACRO
+definition equs "inner_arg: MACRO\nPRINTT \"outer: \1\\ninner: \\1\\n\"\nENDM"
+definition
+	PURGE definition
+ENDM
+
+	outer_arg outside
+	inner_arg inside
+
+
+outer: MACRO
+	WARN "Nested macros shouldn't work, whose argument would be \\1?"
+inner: MACRO
+ENDM
+
+	outer
+	inner