gb-emu: initial commit

Add a mostly non-functional Gameboy CPU and the skeleton
of a Gameboy assembler intended for unit tests.

src/gbasm/errors.h

@@ -0,0 +1,17 @@
+#ifndef GB_ASM_ERRORS_H
+#define GB_ASM_ERRORS_H
+
+#include "gbasm/types.h"
+
+void gbasm_unknown_opcode_error(const struct gbasm_parsed_inst *inst);
+
+void gbasm_too_many_args_error(const struct gbasm_parsed_inst *inst,
+                               const struct gbasm_op_info *opcode);
+
+void gbasm_arg_wrong_type_error(const struct gbasm_parsed_inst *inst,
+                                const struct gbasm_op_info *opcode);
+
+void gbasm_too_few_args_error(const struct gbasm_parsed_inst *inst,
+                              const struct gbasm_op_info *opcode);
+
+#endif