Add payment-channel example (#1248)

* add payment-channel example

* code review changes

* fix typo

* fix typo

* review changes

* review changes

* Clean up invalid `close` test a little

Co-authored-by: Hernando Castano <hernando@hcastano.com>

examples/payment-channel/.gitignore

@@ -0,0 +1,9 @@
+# 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

@@ -0,0 +1,39 @@
+[package]
+name = "payment_channel"
+version = "4.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2021"
+publish = false
+
+[dependencies]
+ink_primitives = { path = "../../crates/primitives", default-features = false }
+ink_metadata = { path = "../../crates/metadata", default-features = false, features = ["derive"], optional = true }
+ink_env = { path = "../../crates/env", default-features = false }
+ink_storage = { path = "../../crates/storage", default-features = false }
+ink_lang = { path = "../../crates/lang", default-features = false }
+
+scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive"] }
+scale-info = { version = "2", default-features = false, features = ["derive"] }
+
+[dev-dependencies]
+hex-literal = { version = "0.3" }
+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",
+]
+
+ink-as-dependency = []