Remove byteorder-dependency

[lukaslueg]

This removes this byteorder-dependency which is not strictly needed since Rust 1.32. Due to a single use of TryInto (which could be avoided) the minimum Rust version is now 1.34.2

As one can see, there is a lot more manual reading/writing involved. I think the change is worth losing a dependency, though.

Fixes #40

[sile]

Thank you for your PR!
I left a minor comment.

[codecov-io]

Codecov Report

Merging #42 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   91.34%   91.42%   +0.07%     
==========================================
  Files          14       14              
  Lines        1283     1294      +11     
==========================================
+ Hits         1172     1183      +11     
  Misses        111      111

Impacted Files	Coverage Δ
src/lib.rs	100% <ø> (ø)	⬆️
src/deflate/encode.rs	96.62% <100%> (+0.02%)	⬆️
src/non_blocking/zlib.rs	95.06% <100%> (+0.25%)	⬆️
src/non_blocking/deflate/decode.rs	86.39% <100%> (+0.24%)	⬆️
src/bit.rs	94.21% <100%> (+0.09%)	⬆️
src/util.rs	91.17% <0%> (+0.26%)	⬆️

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[lukaslueg]

Took liberty for force-push the change, getting rid of TryInto. Notice that there does not seem to be a test that actually verifies modification_time.

[sile]

Notice that there does not seem to be a test that actually verifies modification_time.

Hmm...

Anyway, this PR seems good. Thanks!