Make Channel implement AutoCloseable

[adamw]

Closes #23

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Benchmark results

``` Java: (capacity) (chainLength) Mode Cnt Score Error Units BufferedBenchmark.arrayBlockingQueue 1 N/A avgt 5 1017.007 ± 67.333 ns/op BufferedBenchmark.arrayBlockingQueue 10 N/A avgt 5 193.502 ± 33.597 ns/op BufferedBenchmark.arrayBlockingQueue 100 N/A avgt 5 116.331 ± 3.257 ns/op BufferedBenchmark.channel 1 N/A avgt 5 216.174 ± 45.280 ns/op BufferedBenchmark.channel 10 N/A avgt 5 185.888 ± 38.573 ns/op BufferedBenchmark.channel 100 N/A avgt 5 156.230 ± 20.229 ns/op ChainedBenchmark.channelChain 0 100 avgt 5 1007.035 ± 4.340 ns/op ChainedBenchmark.channelChain 0 1000 avgt 5 1016.213 ± 17.392 ns/op ChainedBenchmark.channelChain 0 10000 avgt 5 1026.876 ± 16.654 ns/op ChainedBenchmark.channelChain 100 100 avgt 5 30.556 ± 0.654 ns/op ChainedBenchmark.channelChain 100 1000 avgt 5 30.479 ± 0.870 ns/op ChainedBenchmark.channelChain 100 10000 avgt 5 31.236 ± 1.589 ns/op ChainedBenchmark.queueChain 0 100 avgt 5 109.663 ± 12.628 ns/op ChainedBenchmark.queueChain 0 1000 avgt 5 125.174 ± 24.305 ns/op ChainedBenchmark.queueChain 0 10000 avgt 5 126.795 ± 38.346 ns/op ChainedBenchmark.queueChain 100 100 avgt 5 10.859 ± 0.638 ns/op ChainedBenchmark.queueChain 100 1000 avgt 5 11.017 ± 0.209 ns/op ChainedBenchmark.queueChain 100 10000 avgt 5 9.971 ± 0.575 ns/op RendezvousBenchmark.channel N/A N/A avgt 5 174.572 ± 33.856 ns/op RendezvousBenchmark.exchanger N/A N/A avgt 5 85.974 ± 15.187 ns/op RendezvousBenchmark.synchronousQueue N/A N/A avgt 5 604.921 ± 171.611 ns/op SelectBenchmark.selectWithSingleClause N/A N/A avgt 5 229.317 ± 63.647 ns/op SelectBenchmark.selectWithTwoClauses N/A N/A avgt 5 209.010 ± 17.063 ns/op Kotlin: (capacity) (chainLength) Mode Cnt Score Error Units BufferedKotlinBenchmark.channel_defaultDispatcher 1 N/A avgt 5 106.512 ± 1.161 ns/op BufferedKotlinBenchmark.channel_defaultDispatcher 10 N/A avgt 5 54.214 ± 0.777 ns/op BufferedKotlinBenchmark.channel_defaultDispatcher 100 N/A avgt 5 32.508 ± 0.719 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 0 100 avgt 5 73.906 ± 3.939 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 0 1000 avgt 5 114.053 ± 124.448 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 0 10000 avgt 5 58.353 ± 4.273 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 100 100 avgt 5 13.970 ± 0.975 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 100 1000 avgt 5 13.848 ± 0.396 ns/op ChainedKotlinBenchmark.channelChain_defaultDispatcher 100 10000 avgt 5 15.135 ± 0.502 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 0 100 avgt 5 102.354 ± 7.999 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 0 1000 avgt 5 155.914 ± 15.449 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 0 10000 avgt 5 94.958 ± 1.334 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 100 100 avgt 5 29.260 ± 0.229 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 100 1000 avgt 5 31.472 ± 0.191 ns/op ChainedKotlinBenchmark.channelChain_eventLoop 100 10000 avgt 5 31.386 ± 0.748 ns/op RendezvousKotlinBenchmark.channel_defaultDispatcher N/A N/A avgt 5 148.933 ± 0.661 ns/op SelectKotlinBenchmark.selectWithSingleClause_defaultDispatcher N/A N/A avgt 5 249.941 ± 14.974 ns/op SelectKotlinBenchmark.selectWithTwoClauses_defaultDispatcher N/A N/A avgt 5 312.930 ± 1.185 ns/op ```