Suggestion: WebAssembly support

Hi,
I was interested in seeing if I could run differential-dataflow on the web by compiling to WebAssembly using wasm-pack and it worked out really well! However it required some hacks changes to differential-dataflow and timely-dataflow, specifically:

• For both packages, replacing std::time::Instant with web_time::Instant - crate: web-time This is because there is no "current time" implemented for the wasm32-unknown-unknown compilation target - so we have to hook it up to the browser's Performance.now() function. In all other contexts this uses the regular std::time::Instant under the hood so shouldn't make any difference.
• Differential dataflow has a 63 bit shift (1 << 63) which needs to be (1 << 31) in 32-bit WebAssembly.

The changes required are:

• Timely dataflow: https://github.com/TimelyDataflow/timely-dataflow/compare/master...oli-w:timely-dataflow:master
• Differential dataflow: https://github.com/TimelyDataflow/differential-dataflow/compare/master...oli-w:differential-dataflow:master

I was wondering if you would be interested in incorporating this or would prefer to keep this kind of change as a fork?

Start by reviewing the linked timely-dataflow and differential-dataflow compare branches, then check the wasm-pack approach and the wasm32-unknown-unknown constraints described in the issue. Done means reaching a maintainer decision and, if accepted, incorporating equivalent WebAssembly support in both packages while preserving non-WebAssembly behavior.