Make the crate no_std-compatible

[mina86]

Even though the crate doesn’t use std crate, because it doesn’t
declare #![no_std] it cannot be compiled in no_std environments:

error[E0463]: can't find crate for `std`
  = note: the `thumbv6m-none-eabi` target may not support the standard library
  = note: `std` is required by `forward_ref` because it does not declare `#![no_std]`

Adding the annotation makes it possible to use the crate on no_std
targets.

Issue: CosmWasm/cosmwasm#1484

[mina86]

@jtempest, friendly ping.