fix bare function imports and update example to use it

[dicej]

Bare (i.e. not contained in an interface) function imports weren't working, so I fixed that, which also required https://github.com/bytecodealliance/wasm-tools/pull/1176 and https://github.com/bytecodealliance/wasmtime-py/pull/176. Also, I forgot to update the matrix-math example to reflect the Wasmtime upgrade in my previous commit, so I've address that, too.