Exclude files from Cargo package

[TehPers]

Closes #488

This excludes unnecessary files from the package that is published to crates.io. This reduces the size of the package to less than half of what it was before by excluding some of the larger files, especially the ones in the tests folder.

The files that are being included in the package as a result of this PR are:

• src/*
• LICENSE-MIT.md
• README.md

Some other files I considered but did not include are:

• examples/* (these can be found online, and rustdoc is still available offline)
• Changelog.md (this file is rather large and may be better suited as an online resource)

To ensure that cargo publish is unaffected by the exclusion of unit tests and benchmarks, paths have been added explicitly to those items in the Cargo.toml file. See this thread for more information.

[codecov-commenter]

Codecov Report

Merging #489 (3ef8b10) into master (240f204) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   54.02%   54.04%   +0.01%     
==========================================
  Files          30       30              
  Lines       12662    12649      -13     
==========================================
- Hits         6841     6836       -5     
+ Misses       5821     5813       -8     

Flag	Coverage Δ
unittests	54.04% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/lib.rs	12.66% <0.00%> (-0.29%)	⬇️
src/escapei.rs	12.10% <0.00%> (-0.06%)	⬇️
src/reader/parser.rs	98.61% <0.00%> (-0.01%)	⬇️
src/de/mod.rs	69.13% <0.00%> (+0.28%)	⬆️
src/reader/buffered_reader.rs	85.84% <0.00%> (+0.47%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.