Fix table index bug

[Gaboose]

Hi @mathetake! First of all, nice work with this project!

I tried running a rust-compiled hello world program with the wasm32-wasi target, started adding the required methods into the wasi package, and was pretty excited to get it working, but then kept getting a function signature mismatch panic from the callIndirect function in vm_call.go. It took me quite a while to debug, and I got to understand the wasm binary format way better because of it, but finally found the culprit:

There's a classic bug in the code that populates the table index space. This loop:

for i, b := range elem.Init {
	table[i+offset] = &b
}

Takes the pointer of the temporary variable, which is reused on every iteration. The fix is just to use table[i+offset] = &elem.Init[i].

Hope you see this pull request, and merge it! I want to contribute some more code to the wasi package afterwards.

[mathetake]

@Gaboose thank you so much! Actually I have been aware of this Go-newbie bug since #16 is submitted, but never got to have a time to fix til now.

LGTM modulo some nits. Thank you!