UTF-8 validation

UTF-8 is a very popular string encoding, for example, it's the encoding used by over 95% of all Web content. It's not uncommon for applications to need to do UTF-8 validation on their own strings, and since all WebAssembly VMs have UTF-8 validation logic built in as required by the spec, we should define a WASI API to let applications call into the VM's UTF-8 validation logic rather than having to bundle their own.

I'm picturing an API which takes a byte slice as input and returns a boolean value indicating whether it's valid or not. This is the minimum that WebAssembly engines themselves are required to have, and would be enough for eg. the use case of implementing a UTF-8 validity check for a a WASI API implemented in wasm.

More elaborate APIs are possible, such as validation which returns the position where an error occurred, and possibly information about the error, but I think it makes sense to start with something simple. I won't have time to make an official proposal myself for a while, but I wanted to file this issue to see what others think!

Start by reviewing the proposed WASI API shape in this issue and the WebAssembly requirement for UTF-8 validation. Compare the simple boolean-returning design with the more elaborate alternatives described here, then determine the proposal, specification, and implementation or test entry points that need to change. Done means the API design is agreed and its behavior is specified and validated.