Make SocketDescriptor::send_data take slice instead of vec and offset

[rrybarczyk]

By taking in a slice, we can avoid forcing the caller to pass in write_offset.

Another added benefit is that you can pass in slices or anything that can be converted to a slice, including a vec.

Also, added the net-tokio target directory to the .gitignore. Can put this a different PR if preferred.

[codecov]

Codecov Report

Merging #369 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
+ Coverage   87.33%   87.34%   +0.01%     
==========================================
  Files          29       29              
  Lines       15605    15605              
==========================================
+ Hits        13628    13630       +2     
+ Misses       1977     1975       -2

Impacted Files	Coverage Δ
src/ln/peer_handler.rs	31.81% <0%> (ø)	⬆️
src/ln/functional_tests.rs	96.38% <0%> (+0.05%)	⬆️

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[TheBlueMatt]

Awesome, looks great! You can tell what bits I wrote before I knew rust very well...Will take as #370 (which just fixed a few indentation things that slipped in here).