Merge #940: Add psbt watch-only/cold-storage example

cd2369b Run ecdsa-psbt example in test script (Tobin C. Harding)
6967c0e Add psbt example (Tobin Harding)

Pull request description:

  Add an example PSBT workflow. The workflow we simulate is that of a setup using a watch-only online wallet (contains only public keys) and a cold-storage wallet (contains the private keys).

  We create and update a PSBT using the watch-only wallet then pass the PSBT to the cold-storage wallet to sign.

  Partially resolves #892 (more done in #935).

  ## Note

  This PR includes a sub-module in `examples/psbt.rs` that implements ECDSA signing. This will hopefully eventually be merged into the main crate by way of #957.  We have three PRs that add examples/tests of PSBTs that need to do signing, in order to help us design a good AP in #957 I think it would be beneficial to complete and merge these three PRs.

  1. This PR (PSBT ECDSA example)
  2. PBST ECDSA integration test: #935
  3. PSBT taproot example: #999

  Note to self, this will need a change to `test.sh` if #1079 merges.

ACKs for top commit:
  apoelstra:
    ACK cd2369b
  sanket1729:
    ACK cd2369b. The example is clean, you can address the comments in followups.

Tree-SHA512: c4fb8ec631bf8bfc30534e8974b1f6c4bb7cc6def165a4ee2bb7aa73f5aa7fdc11d2000ca25792a4b534b2c5bf1592efe542ada14a9d702c7dfacbaa808f3aea

Cargo.toml

@@ -56,3 +56,7 @@ name = "bip32"
 [[example]]
 name = "handshake"
 required-features = ["std"]
+
+[[example]]
+name = "ecdsa-psbt"
+required-features = ["std", "bitcoinconsensus"]

contrib/test.sh

@@ -62,6 +62,8 @@ do
     cargo test --verbose --features="$feature"
 done
 
+cargo run --example ecdsa-psbt --features=bitcoinconsensus
+
 # Build the docs if told to (this only works with the nightly toolchain)
 if [ "$DO_DOCS" = true ]; then
     RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links