implement protocol ast and instruction-selection examples

examples/instructions-incomplete.tess

@@ -0,0 +1,9 @@
+data Exp = Const Int | Reg Int | Plus Exp Exp | Times Exp Exp
+
+data Instr = LoadImm Int | LoadReg Int | AddInstr | MulInstr
+
+match select (e : Exp) : Instr =
+  | Plus (Reg a) (Reg b) -> AddInstr
+  | Times (Reg a) (Reg b) -> MulInstr
+  | Const n -> LoadImm n
+  | Reg n -> LoadReg n