Add WASM support for executor

* Adds an executor for WASM runtimes based on wasm_bindgen.
* Add time driver based on JS time handling.
* Add example that can run in browser locally.
* Update to critical-section version that supports 'std' flag

examples/wasm/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+authors = ["Ulf Lilleengen <lulf@redhat.com>"]
+edition = "2018"
+name = "embassy-wasm-example"
+version = "0.1.0"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+embassy = { version = "0.1.0", path = "../../embassy", features = ["log", "wasm"] }
+
+wasm-logger = "0.2.0"
+wasm-bindgen = "0.2"
+web-sys = { version = "0.3", features = ["Document", "Element", "HtmlElement", "Node", "Window" ] }
+log = "0.4.11"
+critical-section = "0.2.2"