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/filetests/src/test_unwind.rs

@@ -39,10 +39,10 @@ impl SubTest for TestUnwind {
         let isa = context.isa.expect("unwind needs an ISA");
         let mut comp_ctx = cranelift_codegen::Context::for_function(func.into_owned());
 
-        comp_ctx.compile(isa).expect("failed to compile function");
+        let code = comp_ctx.compile(isa).expect("failed to compile function");
 
         let mut text = String::new();
-        match comp_ctx.create_unwind_info(isa).expect("unwind info") {
+        match code.create_unwind_info(isa).expect("unwind info") {
             Some(UnwindInfo::WindowsX64(info)) => {
                 let mut mem = vec![0; info.emit_size()];
                 info.emit(&mut mem);