lang: codegen: redesign codegen to use a stack-based state machine

lang/codegen/generator.c

@@ -0,0 +1,20 @@
+#include "codegen.h"
+
+extern const struct code_generator unit_generator;
+extern const struct code_generator expr_generator;
+
+static const struct code_generator *code_generators[] = {
+	[CODE_GENERATOR_UNIT] = &unit_generator,
+	[CODE_GENERATOR_EXPR] = &expr_generator,
+};
+static const size_t nr_code_generators
+	= sizeof code_generators / sizeof code_generators[0];
+
+const struct code_generator *get_code_generator(enum code_generator_type type)
+{
+	if (type >= nr_code_generators) {
+		return NULL;
+	}
+
+	return code_generators[type];
+}