Add an Encoding meta-language class.

Start adding some RISC-V encodings too as a way of testing the ergonomics.

meta/target/riscv/encodings.py

@@ -0,0 +1,21 @@
+"""
+RISC-V Encodings.
+"""
+from cretonne import base
+from cretonne.types import i32, i64
+from defs import RV32, RV64
+from recipes import OP, R
+
+# Basic arithmetic binary instructions are encoded in an R-type instruction.
+for inst,           f3,    f7 in [
+        (base.iadd, 0b000, 0b0000000),
+        (base.isub, 0b000, 0b0100000),
+        (base.ishl, 0b001, 0b0000000),
+        (base.ushr, 0b101, 0b0000000),
+        (base.sshr, 0b101, 0b0100000),
+        (base.bxor, 0b100, 0b0000000),
+        (base.bor,  0b110, 0b0000000),
+        (base.band, 0b111, 0b0000000)
+        ]:
+    RV32.enc(inst, i32, R, OP(f3, f7))
+    RV64.enc(inst, i64, R, OP(f3, f7))