Approach: Build on top of deflate_init() / inflate_init()

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Refs #1.

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The deflate_init() and inflate_init() functions will be added with PHP 7 (currently undocumented otherwise).

These functions expose exactly what the underlying zlib is capable of and as such would be a perfect base for a streaming (de)compressor.

However, given they will be added with PHP 7 only, compatibility is very limited.

As such, we can't really build this library on top it.
However, we may want to look into how good this performs and could eventually provide two alternative implementations.

FWIW: HHVM's zlib.inflate stream filter (refs #3) uses a class known as __SystemLib_ChunkedInflator which is very similar to what inflate_init() (and family) does. Relying on this is probably not a good idea… ;-)

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The function deflate_init() is currently only available on PHP7+ and has zero support on older versions or HHVM:

https://3v4l.org/pfCii

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Afaict the deflate_*() functions do not exhibit any of the current known inconsistencies with the stream filters (#3):

https://3v4l.org/DtFB2