Use the term "Function parameter" instead of "argument".

Rename the ArgumentType type to AbiParam since it describes the ABI
characteristics of a parameter or return value, not just the value type.

In Signature, rename members argument_types and return_types to "params"
and "returns". Again, they are not just types.

Fix a couple lingering references to "EBB arguments".

lib/cretonne/src/regalloc/spilling.rs

@@ -324,7 +324,7 @@ impl<'a> Context<'a> {
         let args = self.cur.func.dfg.inst_variable_args(inst);
         for (idx, (abi, &arg)) in
             self.cur.func.dfg.signatures[sig]
-                .argument_types
+                .params
                 .iter()
                 .zip(args)
                 .enumerate()