Implement single threaded task scheduler for WebAssembly

[smokku]

This implements a single threaded "TaskPool" and systems scheduler for use with WebAssembly target.

I see it as a temporary solution, until Rust wasm story gets better support for wasm threads (see #88). This is why I kept my change minimal, without any major refactoring. It is ugly, but it works.

Provided examples demonstrate how to build and run wasm32 target. See examples/README.md.

Bevy is now able to run "headless" in browser.

Together with #209 it should be enough (+/- fixes) to run Bevy app in browser.

[smokku]

Should Bevy use +nightly compiled when testing, while it does not require such compiler to run?

[cart]

Yeah it needs nightly for cargo fmt because we currently require "combined imports" (its also required for clippy). im considering dropping that requirement to make the development process a little easier on people.

[smokku]

Oh, it actually did find an improvement to this use line.
I usually use VSCode Refactor->Merge Imports feature for these, but I missed this one. I'm all for keeping it. :-)

Fixed.

[cart]

I'm not going to lie, this put a real smile on my face:

[cart]

The current clippy failure is probably because extern crate console_error_panic_hook probably doesn't work on non-wasm platforms.

Maybe we should create a WasmLogPlugin to make configuring this easier / hiding it behind a feature flag easier?

[smokku]

Maybe we should create a WasmLogPlugin to make configuring this easier / hiding it behind a feature flag easier?

It is not a part of the engine per-se. Just an example on how one could handle panics in own application.
But it is not required.

Anyway, moving it to normal dev-dependencies (not target specific ones) helps.

[cart]

Alrighty lets merge this 😄