Misc cleanups (#5014)

* Replace resize+copy_from_slice with extend_from_slice

Vec::resize initializes the new space, which is wasted effort if we're
just going to call `copy_from_slice` on it immediately afterward. Using
`extend_from_slice` is simpler, and very slightly faster.

If the new size were bigger than the buffer we're copying from, then it
would make sense to initialize the excess. But it isn't: it's always
exactly the same size.

* Move helpers from Context to CompiledCode

These methods only use information from Context::compiled_code, so they
should live on CompiledCode instead.

* Remove an unnecessary #[cfg_attr]

There are other uses of `#[allow(clippy::too_many_arguments)]` in this
file, so apparently it doesn't need to be guarded by the "cargo-clippy"
feature.

* Fix a few comments

Two of these were wrong/misleading:

- `FunctionBuilder::new` does not clear the provided func_ctx. It does
  debug-assert that the context is already clear, but I don't think
  that's worth a comment.

- `switch_to_block` does not "create values for the arguments." That's
  done by the combination of `append_block_params_for_function_params`
  and `declare_wasm_parameters`.

* wasmtime-cranelift: Misc cleanups

The main change is to use the `CompiledCode` reference we already had
instead of getting it out of `Context` repeatedly. This removes a bunch
of `unwrap()` calls.

* wasmtime-cranelift: Factor out uncached compile

cranelift/codegen/src/isa/x64/inst/unwind/systemv.rs

@@ -118,9 +118,9 @@ mod tests {
             Some(StackSlotData::new(StackSlotKind::ExplicitSlot, 64)),
         ));
 
-        context.compile(&*isa).expect("expected compilation");
+        let code = context.compile(&*isa).expect("expected compilation");
 
-        let fde = match context
+        let fde = match code
             .create_unwind_info(isa.as_ref())
             .expect("can create unwind info")
         {
@@ -157,9 +157,9 @@ mod tests {
 
         let mut context = Context::for_function(create_multi_return_function(CallConv::SystemV));
 
-        context.compile(&*isa).expect("expected compilation");
+        let code = context.compile(&*isa).expect("expected compilation");
 
-        let fde = match context
+        let fde = match code
             .create_unwind_info(isa.as_ref())
             .expect("can create unwind info")
         {