Refactoring and Updates

[robertjpayne]

This implements several discussed changes to Venice:

1. Bundles libdill directly, this avoids the need to use an apt/brew package
2. Adds coroutine local storage support
3. Refactors coroutines so they manage their own lifetime and clean up automatically

[codecov[bot]]

Codecov Report

Merging #40 into swift-4 will decrease coverage by 19.4%. The diff coverage is 54.72%.

@@             Coverage Diff             @@
##           swift-4     #40       +/-   ##
===========================================
- Coverage       91%   71.6%   -19.41%     
===========================================
  Files            5       5               
  Lines          456     479       +23     
  Branches        22      23        +1     
===========================================
- Hits           415     343       -72     
- Misses          41     136       +95

Impacted Files	Coverage Δ
Sources/Venice/FileDescriptor.swift	89.84% <ø> (-1.7%)	:arrow_down:
Sources/Venice/Error.swift	70% <100%> (-5%)	:arrow_down:
Sources/Venice/Coroutine.swift	43.11% <43.35%> (-49%)	:arrow_down:
Sources/Venice/Channel.swift	81.51% <82.45%> (-8%)	:arrow_down:

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[paulofaria]

The drop in coverage is because of libdill? Let me see if we can ignore it.