In an effort to improve the documentation on `bitcoin_io` add an example
in `bitcoin` crate that demonstrates a few things:
- Encode/Decode a `rust-bitcoin` type to/from a stdlib type.
- Encode to a custom type by implementing `bitcoin_io` traits.
- Encode to a foreign custom type by using the `bitcoin_io::bridge::FromStd` wrapper.
Later we can link to this example online in the `bitcoin_io` docs.
Tobin C. Harding