Expose UTF-8 validated string type

[tgnottingham]

bytes has become a little bit of a de facto standard for low/zero-copy techniques, with authors of some popular crates being comfortable with exposing bytes types in their interfaces. In some cases, those crates would benefit from a bytes string wrapper to enable zero-copy string handling, but they don't support such handling because bytes doesn't and they don't want to include another crate like bytestring, which isn't a de facto standard as of now, in their interfaces.

For example, prost, which implements conversion between Rust types and protocol buffers, supports using Bytes instead of Vec<u8> to back protobuf bytes fields, but it doesn't a similar alternative for String, despite it being a requested feature (tokio-rs/prost#752), in part because bytes doesn't have a string wrapper type.

Would the bytes maintainers be open to exposing such a type, along the lines of the one described in #373, in order to better enable these use cases in the ecosystem?