Add psbt example

Draft because I'm stuck.

We are currently working on PSBT workflow that uses the test vectors
from BIP174, issue #892 describes the problem.

PR #935 implements a solution using the test vectors but the code is
cluttered because of all the hex strings, also the code does not really
show how a user of `rust-bitcoin` would go about using the `psbt`
module.

I propose that #935 goes into `src/tests`, this PR  adds
`src/examples/psbt.rs`.

Add an example PSBT workflow based on a cold-storage signer and a
watch-only online wallet. The watch-only wallet is the Creator, Updater,
and Finalizer. The cold-storage wallet is the Signer.

Cargo.toml

@@ -58,3 +58,6 @@ name = "bip32"
 [[example]]
 name = "handshake"
 required-features = ["std"]
+
+[[example]]
+name = "psbt"