Ensure ABI functions properly cast ints

[avidal]

So, wasm has int32 and int64 and makes no decision as to signed or unsigned, that part is left up to the compiler/runtime. wasmtime chose to use unsigned, which means all of the import funcs (functions defined in Go that are made available to the wasm program) have signatures that take a number of uint32 and uint64 args and return a uint32.

But, some of those uints should be cast to signed and there may be one or more bugs hiding in there as a result.

I'll need to go over each ABI method signature again in fastly-sys and ensure the types we use in fastlike match the types used on the fastly side to prevent an under/overflow.

For instance, "handle" types (request, response, body) are all uint32 which means we're safe, but functions like xqd_body_write which expect the embedder to write the number of bytes written to a pointer in linear memory expect the numbers of bytes written to be usize