support destructors for imported resources

[dicej]

This provides partial support for #68. I've focused on imported resources to begin with since APIs such as wasi:http make heavy use of child handles, which must be disposed in the correct order to avoid traps.

Supporting destructors for exported resources may also be useful, but not as urgent with respect to WASI.

[dicej]

Here's an example: https://github.com/fermyon/spin-fileserver/blob/main/examples/javascript/app.mjs