Replace all `String::as_bytes()` and `from_utf8()` calls with Postcard

[quietlychris]

Although Postcard is being used for serialization, there are some cases where we are simply using the UTF-8 encoding for Strings. This is inconsistent, and should be changed so that any byte-level representation of data happens using the Postcard serialization schema. This is important for potentially supporting other serde-compatible serializers in the future.