Make nannou package compile in WASM environment

[Woyten]

Hi. This is a minimal example showing which changes will allow to compile the nannou module against the wasm32-unknown-unknown target.

Disclaimer: I am not saying that:

• Nannou Apps will run in a Browser after these changes. There are still some things to do.
• This PR should be merged. But it could! 😉

This PR relates to #475.

[mitchmindtree]

Nice to see a start on this!

One thing that might be worth adding while landing wasm support would be one new CI job that tries to build one or more of the examples for the wasm target, or something along these lines. Normally I would say CI testing of specific platforms is best left for the cross platform crates like winit and cpal. However in our case I think it might be easy for us to accidentally break wasm compatibility otherwise, particularly as some library APIs just aren't compatible with wasm (like those wgpu items you had to gate in this PR) and it's not always obvious what those are.

[Woyten]

Sorry I haven't responded yet. I will rebase this PR and see how to adapt the CI job once #737 is merged.

[Woyten]

@mitchmindtree I upgraded this PR to the newest Nannou version and added a CI check.

[Woyten]

@mitchmindtree Currently, it's not possible to compile the whole Nannou tree to WASM. nannou_timeline depends on envelope which depends on num which depends on rustc-serialize. rustc-serialize will not receive WASM support (rust-lang-deprecated/rustc-serialize#190 (comment)). This means, a new envelope version without the implicit rustc-serialize dependency is needed.

[Woyten]

Other problematic libraries that need to be removed or hidden behind a feature switch are socket2 and home.

[Woyten]

@mitchmindtree The PR has been updated to the newest Nannou release. I propose to merge the PR s.t. the main nannou package doesn't lose WASM compatibility in the future.