add transport and cleanup

[tanner0101]

• add Transport to socks
• add readable and writeable streams
• cleanup Stream conformance (redundant classes removed)
• made SecurityLayer generic so TLS isn't required at this layer

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by 5.77%. The diff coverage is 67.01%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   62.44%   68.22%   +5.77%     
==========================================
  Files          27       37      +10     
  Lines        1161     1501     +340     
==========================================
+ Hits          725     1024     +299     
- Misses        436      477      +41

Impacted Files	Coverage Δ
Sources/Sockets/Core/Conversions.swift	100% <ø> (ø)
Tests/SocketsTests/ConversionTests.swift	100% <ø> (ø)
Tests/SocketsTests/XCTestManifests.swift	0% <ø> (ø)
Sources/Sockets/Core/Types.swift	77.5% <0%> (ø)
Sources/Sockets/TCP/TCPEstablishedSocket.swift	100% <100%> (ø)
Tests/SocketsTests/PipeTests.swift	100% <100%> (ø)
...ransport/Streams/ProgramStream/SecurityLayer.swift	100% <100%> (ø)
Sources/Sockets/Core/Buffer.swift	100% <100%> (ø)
Tests/SocketsTests/SelectTests.swift	100% <100%> (ø)
Tests/SocketsTests/LifetimeTests.swift	81.25% <100%> (ø)
... and 48 more

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