Ehud Shapiro's promise based Queue in the browser

To test my web_send promise implementation, I made an AJAX interface
to the weird promise based Queue implementation that supports popping
values before they've been added. The browser has to poll for the
resolved value of a promise, so be a little patient if you let the
page sit for a while before interacting with it (the polling slows
down the longer it's been waiting for).

See:
https://vsci.hpl.hp.com/-/series/#s=lm2fyc6lq755yu

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