Using vendored copy of zlib SwiftPM package rather than system copy of zlib.

[satishbabariya]

Discussion on which solution is a better solution,

1. vendored copy of zlib SwiftPM package
2. system copy of zlib.

as @Lukasa proposed using a separate SwiftPM package that is appropriately versioned and then add an appropriate dependency on it.
rather than integrating vendored copy of zlib into the swift-nio directly (-1 form @Lukasa)

if swift-nio 2.0 goes with the vendored copy of zlib SwiftPM package, then the swift-nio adopters can use same SwiftPM package.

[Lukasa]

To be clear, I am also opposed to depending on a SwiftPM zlib package. I think the most interesting alternative would be a well-maintained Swift implementation of gzip and deflate.

Until that time I think I’d rather use the system zlib.

[weissi]

@satishbabariya so we will actually move the HTTP compressor (which requires this dependency) into swift-nio-extras

[satishbabariya]

Yeah, that would be great also.

[firecrackerz]

@weissi there are some pure swift implementation i found, it's worth looking into.
https://github.com/tsolomko/SWCompression

It also works both on Apple platforms and Linux.

All features are listed in the tables below.
"TBD" means that feature is planned but not implemented (yet).

	Deflate	BZip2	LZMA/LZMA2
Decompression	✅	✅	✅
Compression	✅	✅	TBD

	Zlib	GZip	XZ	ZIP	TAR	7-Zip
Read	✅	✅	✅	✅	✅	✅
Write	✅	✅	TBD	TBD	✅	TBD