Use the term "EBB parameter" everywhere.

Add EBB parameter and EBB argument to the langref glossary to clarify
the distinction between formal EBB parameter values and arguments passed
to branches.

- Replace "ebb_arg" with "ebb_param" in function names that deal with
  EBB parameters.
- Rename the ValueDef variants to Result and Param.
- A bunch of other small langref fixes.

No functional changes intended.

lib/cretonne/src/ir/function.rs

@@ -150,7 +150,7 @@ impl Function {
     pub fn special_arg(&self, purpose: ir::ArgumentPurpose) -> Option<ir::Value> {
         let entry = self.layout.entry_block().expect("Function is empty");
         self.signature.special_arg_index(purpose).map(|i| {
-            self.dfg.ebb_args(entry)[i]
+            self.dfg.ebb_params(entry)[i]
         })
     }
 }