Generate binemit::emit_inst() functions.

Use the meta language encoding recipes to generate an emit_inst()
function for each ISA. The generated calls into recipe_*() functions
that must be implemented by hand.

Implement recipe_*() functions for the RISC-V recipes.

Add the TargetIsa::emit_inst() entry point which emits an instruction to
a CodeSink trait object.

lib/cretonne/src/ir/valueloc.rs

@@ -25,6 +25,14 @@ impl Default for ValueLoc {
 }
 
 impl ValueLoc {
+    /// Get the register unit of this location, or panic.
+    pub fn unwrap_reg(self) -> RegUnit {
+        match self {
+            ValueLoc::Reg(ru) => ru,
+            _ => panic!("Expected register: {:?}", self),
+        }
+    }
+
     /// Return an object that can display this value location, using the register info from the
     /// target ISA.
     pub fn display<'a, R: Into<Option<&'a RegInfo>>>(self, regs: R) -> DisplayValueLoc<'a> {