Add JsValue::{from_serde, into_serde}

These functions are activated with the `serde-serialization` feature of the
`wasm-bindgen` crate. When activated they will allow passing any arbitrary value
into JS that implements the `Serialize` trait and receiving any value from JS
using the `Deserialize` trait. The interchange between JS and Rust is JSON.

Closes #96

Author: alexcrichton

Cargo.toml

@@ -20,9 +20,12 @@ doctest = false
 default = ["spans", "std"]
 spans = ["wasm-bindgen-macro/spans"]
 std = []
+serde-serialize = ["serde", "serde_json", "std"]
 
 [dependencies]
 wasm-bindgen-macro = { path = "crates/macro", version = "=0.2.6" }
+serde = { version = "1.0", optional = true }
+serde_json = { version = "1.0", optional = true }
 
 [dev-dependencies]
 wasm-bindgen-cli-support = { path = "crates/cli-support", version = '=0.2.6' }