refactor channels.trySend/tryRecv and improve tests #207

Job	Run time
linux-amd64-nim-devel (master)	49s
linux-amd64-nim-version-2-0 (master)	5m 32s
linux-amd64-nim-version-1-6 (master)	36s
macos-amd64-nim-devel (master)	4m 41s
macos-amd64-nim-version-2-0 (master)	5m 3s
macos-amd64-nim-version-1-6 (master)	3m 53s
windows-amd64-nim-devel (master)	6m 32s
windows-amd64-nim-version-2-0 (master)	4m 17s
windows-amd64-nim-version-1-6 (master)	6m 26s
windows-i386-nim-devel (master)	6m 28s
windows-i386-nim-version-2-0 (master)	4m 36s
windows-i386-nim-version-1-6 (master)	6m 38s
	55m 31s

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