Change the default stubbing value to 0

[arnaudgolfouse]

We currently use 71 as the default return value for functions stubbed by wasi-stub. This was done to call attention to a somewhat weird value, so that someone might realize they called a stubbed function.

However, people probably want the stubbed calls to succeed anyways, because they can’t avoid it (e.g. the call happen in a library). Recently it was brought up that the default hasher of HashMap suffers from this…

Was someone actually helped by the 71 value ?