Updating request library to mimic fetch API

[rorticus]

This PR updates the request APIs to mimic the fetch APIs, which makes it possible to do cool stuff like look at the response headers before the body is downloaded.

This also introduces an event based Response object that can be used to monitor the download progress and send responses to streams (a helper utility for that will be provided in dojo-streams).

Issue: https://github.com/dojo/core/issues/253

[codecov-io]

Codecov Report

Merging #261 into master will decrease coverage by -1.64%.

@@           Coverage Diff            @@
##           master   #261      +/-   ##
========================================
- Coverage   95.63%    94%   -1.64%     
========================================
  Files          34     37       +3     
  Lines        1900   2167     +267     
  Branches      373    410      +37     
========================================
+ Hits         1817   2037     +220     
- Misses         25     52      +27     
- Partials       58     78      +20

Impacted Files	Coverage Δ
src/text.ts	82.75% <100%> (+0.3%)	:white_check_mark:
src/has.ts	100% <100%> (ø)	:white_check_mark:
src/QueuingEvented.ts	100% <100%> (ø)
src/request/util.ts	66.66% <33.33%> (-22.23%)	:x:
src/request/Headers.ts	76.92% <76.92%> (ø)
src/request/Response.ts	81.81% <81.81%> (ø)
src/request/TimeoutError.ts	85.71% <85.71%> (ø)
src/request/providers/xhr.ts	88.27% <88.27%> (ø)
src/request/providers/node.ts	89.1% <89.1%> (ø)
src/request/ProviderRegistry.ts	92.85% <92.85%> (ø)
... and 10 more

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[agubler]

I don't think we have anything dojo 2 using request, so i'm also okay with the changes. See you've asked for a review from @bryanforbes, that's probably a good idea 👍