remove utils introduced in #12

[bicycle1885]

I'd like to propose that removing utility functions introduced in #12: writegz, readgz, and gzopen.
There are some reasons I don't think these are suitable in Libz.jl:

1. Libz.jl is an interface package for zlib, which doesn't offer these functions.
2. Accessing a compressed file by its filename is not a standard way: Neither zlib of Python3 nor Zlib of Node.js does not have such interfaces.
3. These functions are fairly easy to implement; the user can define these things with a few lines of code if necessary.

Any thoughts?

[codecov-io]

Current coverage is 75.28%

Merging #19 into master will decrease coverage by -3.32% as of 184c925

@@            master     #19   diff @@
======================================
  Files            5       5       
  Stmts          187     178     -9
  Branches         0       0       
  Methods          0       0       
======================================
- Hit            147     134    -13
  Partial          0       0       
- Missed          40      44     +4

Review entire Coverage Diff as of 184c925

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[bicycle1885]

I will apply this change before the next release if no objections.

[TransGirlCodes]

I think these are reasonable arguments, and that this is all good.

[bicycle1885]

Thanks. I will merge this if tests are green.

[bicycle1885]

Environment: JULIAVERSION=julianightlies/bin/winnt/x64/julia-latest-win64.exe hangs on AppVeyor. This sometimes happens, and would be harmless because other tests have passed.