Begin source generation.

Start out easy by emiting an opcodes.rs file containing an opcode enumeration.

meta/build.py

@@ -3,9 +3,15 @@
 # This script is run from src/libcretonne/build.rs to generate Rust files.
 
 import argparse
+import target
+import gen_instr
 
 parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
 parser.add_argument('--out-dir', help='set output directory')
 
 args = parser.parse_args()
 out_dir = args.out_dir
+
+targets = target.all_targets()
+
+gen_instr.generate(targets, out_dir)