mie: add value type hierarchy definitions

every construct within Mie, such as constants, instructions, functions, and translation units, are sub-types of the generic mie_value struct.

mie_value will facilitate iterating through the IR, as well as converting the IR to/from different formats.

mie/include/mie/instr.h

@@ -0,0 +1,17 @@
+#ifndef MIE_INSTR_H_
+#define MIE_INSTR_H_
+
+#include <mie/value.h>
+
+enum mie_instr_type {
+	MIE_INSTR_NONE = 0,
+};
+
+struct mie_instr {
+	struct mie_value i_base;
+	enum mie_instr_type i_type;
+
+	b_queue i_operands;
+};
+
+#endif