Wasmer runtime panics with metering middleware #183

This issue may be a consequence of how fp-bindgen is design vs. a real bug, but I figured may as well report it here.

Wasmer's metering middleware allows you to assign a point cost to different wasm operators, specify an initial point balance for an instance, and then error when the balance is exhausted. This functionality is useful for blockchains or other environments that want to limit resources spent during the execution of some exported function.

I was playing around with the functionality and noticed that instead of erroring when an instance's metering points are exhausted, fp-bindgen just panics.

Here's a fork just adds the metering middleware to the example-protocol bindings default_store method: https://github.com/sanderpick/fp-bindgen/blob/sander/metering-panic/examples/example-protocol/bindings/rust-wasmer-runtime/bindings.rs#L54 and sets a very low initial metering point value. Note that I committed the bindings directory in order to point to it here.

When you run the primitives test, you'll notice a panic from EXC_BAD_INSTRUCTION (code=1, subcode=0xd4a00000) that happens in the exported init function (specifically at function.call()?:

If you comment out the call to init in the tests (https://github.com/sanderpick/fp-bindgen/blob/sander/metering-panic/examples/example-rust-wasmer-runtime/src/test.rs#L280), and then run the fetch_async_data test (https://github.com/sanderpick/fp-bindgen/blob/sander/metering-panic/examples/example-rust-wasmer-runtime/src/test.rs#L259), you'll notice that in this case the panic happens at the get_unchecked call in fp-bindgen-support (https://github.com/sanderpick/fp-bindgen/blob/sander/metering-panic/fp-bindgen-support/src/host/runtime.rs#L37).

If you set the initial metering points to something very high (https://github.com/sanderpick/fp-bindgen/blob/sander/metering-panic/examples/example-protocol/bindings/rust-wasmer-runtime/bindings.rs#L52), say 100_000_000, everything works as expected. You can query for spent points after a call to an exported function, etc. So, it seems like the issue is related to a mishandling of or ignoring of a MiddlewareError (https://github.com/wasmerio/wasmer/blob/447c2e3a152438db67be9ef649327fabcad6f5b8/lib/compiler/src/error.rs#L55).

I've done some digging but wanted to ask here if middlewares might just not work well w/ fp-bindgen.