Prevent duplicate lang items

Rust does not allow duplicate lang items.  When compiled without the
`std` feature, `sr-io` defines two lang items.  Therefore, `sr-io`
compiled without `feature = "std"` must not be linked with `std`.

However, `pwasm-utils` and `wasmi-validation` both bring in `std` unless
compiled with `default-features = "false"`.  This caused a duplicate
lang item error.  Building both with `default-features = "false"`
prevents this error.  When building with `feature = "std"`, they should
both be built with the `std` feature, so this feature needs to be
explicitly depended on.