Simplify Ebb parameter creation.

Ebb parameters are appended explicitly by whoever calls create_block,
so they don't need to also be inferred from branches. This makes the
frontend code more flexible for producers that want to create things
in a different order, and it eliminates more temporary allocations.

FunctionBuilder no longer maintains its own list of the function
parameter values; these can be obtained from the ebb parameter list
on the entry block.

lib/frontend/src/lib.rs

@@ -75,11 +75,12 @@
 //!         builder.declare_var(x, I32);
 //!         builder.declare_var(y, I32);
 //!         builder.declare_var(z, I32);
+//!         builder.append_ebb_params_for_function_params(block0);
 //!
 //!         builder.switch_to_block(block0);
 //!         builder.seal_block(block0);
 //!         {
-//!             let tmp = builder.param_value(0);
+//!             let tmp = builder.ebb_params(block0)[0]; // the first function parameter
 //!             builder.def_var(x, tmp);
 //!         }
 //!         {