Add payment-channel example

examples/payment_channel/.gitignore

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+# Ignore build artifacts from the local tests sub-crate.
+/target/
+
+# Ignore backup files creates by cargo fmt.
+**/*.rs.bk
+
+# Remove Cargo.lock when creating an executable, leave it for libraries
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock

examples/payment_channel/Cargo.toml

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+[package]
+name = "payment_channel"
+version = "3.0.1"
+authors = ["Parity Technologies <[email protected]>"]
+edition = "2021"
+publish = false
+
+[dependencies]
+ink_primitives = { version = "3.0.1", path = "../../crates/primitives", default-features = false }
+ink_metadata = { version = "3.0.1", path = "../../crates/metadata", default-features = false, features = ["derive"], optional = true }
+ink_env = { version = "3.0.1", path = "../../crates/env", default-features = false }
+ink_storage = { version = "3.0.1", path = "../../crates/storage", default-features = false }
+ink_lang = { version = "3.0.1", path = "../../crates/lang", default-features = false }
+
+eth = { package="ink_eth_compatibility", version = "3.0.1", path="../../crates/eth_compatibility", default-features = false }
+hex-literal = { version = "0.3" }
+
+scale = { package = "parity-scale-codec", version = "3.1.2", default-features = false, features = ["derive"] }
+scale-info = { version = "2.1.1", default-features = false, features = ["derive"] }
+
+sp-core = { version = "6.0.0", default-features = false }
+
+[lib]
+name = "payment_channel"
+path = "lib.rs"
+crate-type = ["cdylib"]
+
+[features]
+default = ["std"]
+std = [
+    "ink_metadata/std",
+    "ink_env/std",
+    "ink_storage/std",
+    "ink_primitives/std",
+    "scale/std",
+    "scale-info/std",
+	"sp-core/std",
+    "eth/std",
+]
+
+ink-as-dependency = []

examples/payment_channel/README.md

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+# Payment Channel Example
+
+## What is this example about?
+
+It demonstrates a payment channel in ink!.
+
+The implementation is based on [this post](https://programtheblockchain.com/posts/2018/03/02/building-long-lived-payment-channels/)
+which has an implementation in Solidity.
+
+## On-Chain Deployment
+
+`ink_env::ecdsa_recover()` uses an [unstable interface](https://github.com/paritytech/substrate/tree/master/frame/contracts#unstable-interfaces) 
+of the contracts pallet. The unstable interfaces needs to be enabled for it to work on-chain.