message: rust: add usage example in rustdoc
diff: // Copyright 2019-2020 The Fizzy Authors.
  // SPDX-License-Identifier: Apache-2.0
  -//! This is a Rust interface to [Fizzy](https://github.com/wasmx/fizzy), a WebAssembly virtual machine.
  +//! This is a Rust interface to [Fizzy](https://github.com/wasmx/fizzy), a fast, deterministic, and pedantic WebAssembly interpreter.
  +//!
  +//! # Examples
  +//!
  +//! This is a generic example for parsing and instantiating a module, and executing a simple function with inputs and an output.
  +//!
  +//! ```
  +//! extern crate fizzy;
  +//!
  +//! fn main() {
  +//! // This wasm binary exports a single sum(u32, u32) -> u32 function.
  +//! let wasm = [
  +//! 0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x07, 0x01, 0x60, 0x02, 0x7f,
  +//! 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07, 0x07, 0x01, 0x03, 0x73, 0x75, 0x6d,
  +//! 0x00, 0x00, 0x0a, 0x09, 0x01, 0x07, 0x00, 0x20, 0x00, 0x20, 0x01, 0x6a, 0x0b,
  +//! ];
  +//! let module = fizzy::parse(&wasm).expect("parsing failed");
  +//! let mut instance = module.instantiate().expect("instantiation failed");
  +//! let result = instance
  +//! .execute(
  +//! "sum",
  +//! &[fizzy::TypedValue::U32(42), fizzy::TypedValue::U32(24)],
  +//! 0,
  +//! )
  +//! .expect("execution failed");
  +//! let result = result
  +//! .value()
  +//! .expect("return value expected")
  +//! .as_u32()
  +//! .expect("u32 expected as a return type");
  +//! assert_eq!(result, 66);
  +//! }
  +//! ```
  mod sys;
  