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/flowgraph.rs

@@ -185,7 +185,7 @@ mod tests {
     fn branches_and_jumps() {
         let mut func = Function::new();
         let ebb0 = func.dfg.make_ebb();
-        let cond = func.dfg.append_ebb_arg(ebb0, types::I32);
+        let cond = func.dfg.append_ebb_param(ebb0, types::I32);
         let ebb1 = func.dfg.make_ebb();
         let ebb2 = func.dfg.make_ebb();