Implement resource converters

Hi --

I'm trying to create a resource in a component that can be constructed from values of different types. I could use a variant as an argument, but it would be nice if I could overload the constructor of a resource somehow so that idiomatic ways to construct a resource from various different types could be provided.

For instance, if we could do something like this:

resource blob {
    from(string);
    from(list<u8> as byte-array);
    from(types.descriptor);
    from(types.input-stream);
    from(types.output-stream);
    ...
}

This could generate something like this in the guest trait (or whatever the equivalent, of course):

pub trait GuestBlob: 'static {
    fn from_string(String) -> Blob;
    fn from_byte_array(Vec<u8>) -> Blob;
    fn from_descriptor(filesystem::types::Descriptor) -> Blob;
    ...
}

Then these convenience trait implementations (or whatever equivalent in another language, even if that means nothing as one could use the static methods as they are) would be present:

impl From<String> for Blob {
    fn from(other: String) -> Self {
        Self::from_string(other)
    }
}

impl From<Vec<u8>> for Blob {
    ...
}

impl From<filesystem::types::Descriptor> for Blob {
    ...
}

...

Thoughts on this?

The issue names no files, tests, or entry points; begin by reviewing the component-model resource and type-conversion design discussions. Done means reaching a decided specification for resource conversion syntax and its generated guest-language behavior, with implementation locations and validation work identified.