The `wasmer_export_func_params_arity` and
`wasmer_export_func_returns_arity` functions store the arity in a
`uint32_t`. The `wasmer_instance_call` expects `c_int`. There is a
type mismatch here. It's not annoying in C or C++, but in some other
languages that have bindings to C/C++, it can imply useless casting.
This patch changes `wasmer_instance_call` to expect `uint32_t` for
`params_len` and `results_len` to match the
`wasmer_export_func_*_arity` functions.
