With the new `bitcoin_io` library, implementing `io::Write`
manually is somewhat tricky - for `std` users we really want to
provide an `std::io::Write` implementation, however for `no-std`
users we want to implement against our internal trait.
Sadly we cannot provide a blanket implementation of
`std::io::Write` for all types whcih implement our `io::Write`
trait as its an out-of-crate impl.
Instead, we provide a macro which will either implement
`std::io::Write` or our `io::Write` depending on the feature flags
set on `bitcoin_io`.
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