Add Binaryen's `wasm-opt` to `cargo contract build`

Created by: Robbepop

After merging https://github.com/paritytech/ink/pull/312 ink! smart contracts will suffer from bloated Wasm binaries due to missing optimizations (or bugs?) between Rust and Wasm translation.

These cause a 0x00 16kB buffer to not be removed from the Wasm binary. However, Binaryen's wasm-opt with -O3 (or even -O2) can easily do this among other neat optimization routines.

Some contracts have been shown to have a reduced Wasm binary size from 27kB down to 9kB just through using wasm-opt.

The cargo contract build process should include wasm-opt by default, however, if wasm-opt does not exist on the user machine it should simply avoid using it (as it is been done today) and print the user a warning that the resulting Wasm binary might not be optimal and refer the user to install the wasm-opt tool.