Update to wit-bindgen 0.21. (#247)

Among other things, this pulls in bytecodealliance/wit-bindgen#878,
which fixes a bug I'm hitting when using cargo component with wasi-http.

Author: sunfishcode

Cargo.lock

@@ -98,8 +98,8 @@ rpassword = "7.3.1"
 futures = "0.3.30"
 bytes = "1.5.0"
 which = "6.0.0"
-wit-bindgen-rust = "0.20.0"
-wit-bindgen-core = "0.20.0"
+wit-bindgen-rust = "0.21.0"
+wit-bindgen-core = "0.21.0"
 tempfile = "3.10.1"
 assert_cmd = "2.0.14"
 predicates = "3.1.0"