extend the lifetime of borrowed resource get

[chenyan2002]

I have two use cases to call resource::as_ptr() outside of the wit-bindgen module. So making them public but hidden from doc.

Two use cases:

1. I need a way to convert ResourceBorrow<'a> to &'a Resource. Essentially an owned version of ResourceBorrow::get.
2. When adding some traits impl to resource types, I need to use rep as an identity to distinguish different resources. I could get around by using get(), but it's more direct to use as_ptr().

[alexcrichton]

Thanks! For this I'd prefer to have one of (a) leave them private and users use the &-returning methods, or (b) make them public but document them and explain why they might be used. I don't mind either one, but I'd prefer to not be in the current state where they're public-but-hidden as that feels a bit too brittle to me (e.g. easy to forget why they were public in the first place)

[chenyan2002]

I have a better fix now. The original get function elides the lifetime, so we have fn get(&'b self) -> &'b T. But since Borrow<'a> is valid for the whole 'a duration, we should be able to borrow &'a T instead. The new signature is also semantically more accurate.