Wait, no DecodeWriter?

Well, this PR is entitled "Streaming encoder"... ;) One thing at a time. Most of this logic can eventually be shared with a Write that encodes, and there will also eventually be Read impls as well.

@nox does this suit your use case?

Yes!

Codecov Report

Merging #74 into master will decrease coverage by 0.68%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   97.31%   96.63%   -0.69%     
==========================================
  Files          10       12       +2     
  Lines        1044     1368     +324     
==========================================
+ Hits         1016     1322     +306     
- Misses         28       46      +18

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Yea!

This may be broken.

servo/servo#22147 (comment)

Looking into it

@marshallpierce found out that the problem is probably in the png crate, which uses io::Write::write without checking how many bytes were actually written.

I've opened image-rs/image-png#94. There is no issue with the base64 crate AFAICT.