Reorganize the asm parser to be more like the linker script parser

Split the declarations into those required for the `%union` and
those only required for the `code`.
Only declare functions on top; define them at the bottom.

include/asm/format.hpp

@@ -4,8 +4,6 @@
 #define RGBDS_FORMAT_SPEC_H
 
 #include <stdint.h>
-#include <variant>
-#include <vector>
 
 enum FormatState {
 	FORMAT_SIGN,    // expects '+' or ' ' (optional)
@@ -30,11 +28,6 @@ struct FormatSpec {
 	bool valid;
 };
 
-struct StrFmtArgList {
-	char *format;
-	std::vector<std::variant<uint32_t, char *>> *args;
-};
-
 FormatSpec fmt_NewSpec(void);
 bool fmt_IsEmpty(FormatSpec const *fmt);
 bool fmt_IsValid(FormatSpec const *fmt);