Use integrated wasm-opt

[brson]

Compile wasm-opt and call it as a library instead of as an external binary.

This PR has three patches that result in completely removing support for the wasm-opt CLI in favor of the crate. If desired, the third commit can be excluded, leaving support for both methods.

Documentation for the wasm-opt bindings:

• https://github.com/brson/wasm-opt-rs
• https://docs.rs/wasm-opt/latest/wasm_opt/

Things to be aware of about the wasm-opt crate:

• It depends on Rust 1.48+
• It depends on a c++17 compiler, and should emit a readable error if one is not available.
• The wasm-opt-sys crate takes a non-negligible amount of time to build (minutes on my underpowered laptop). It also does not do any incremental recompilation, so if the build is invalidated it will rebuild the C++ code from scratch. The lack of incremental recompilation is a limitation self-imposed by not using cmake or other external build system.
• wasm-opt on windows does not support extended unicode paths (probably anything non-ascii, maybe latin-1, it's unclear). This is a limitation of binaryen and not a regression of the bindings. It may or may not be fixed in the future. The API will return an error if this occurs.
• cargo tarpaulin (code coverage) segfaults running any wasm-opt crates (Investigate tarpaulin segfault brson/wasm-opt-rs#59), reason unknown. This behavior could infect other crates that link to wasm-opt. If you use tarpaulin, you might check it.

Fixes #733

[cla-bot-2021]

User @brson, please sign the CLA here.

[ascjones]

This is absolutely fantastic thank you @brson, it removes some major friction from our installation process!

It depends on a c++17 compiler, and should emit a readable error if one is not available.

I wonder whether we should include that as a prerequisite. Which reminds me, we should remove Step 2: install binaryen from https://github.com/paritytech/cargo-contract#installation

[brson]

I have updated the readme to remove the binaryen installation step, and added an entry to the changelog.

I am not sure where in the readme to mention the c++17 prerequisite.

[brson]

I've also added a mention of c++17 to the installation section.

[cmichi]

Is it not possible to prefix with #[cfg(target_os = "windows")] here?

[brson]

I don't think so. This is intentionally doing the opposite - making it look like which is used on not-windows. cargo-contract has a clippy lint turned on to check that linked crates are actually used, and which is only used on windows, so clippy doesn't see it on unix. What could be done is to change the Cargo.toml to only link to which on windows.

[cmichi]

This is a great achievement, that hopefully will also benefit the broader Rust+Wasm ecosystem.

Thanks a lot, @brson!