Revert "Many testing tweaks."

This reverts commit 82abf6c491e526b953474415906008b9d12244f5.
Eliminated the return value on the '<-' operator.

It wasn't very useful, and it causes problems if SenderType happens to be a struct
Protocol adjustments; notably ChannelType and SenderType now return whether sending to the channel was successful.

ChannelType's associated type name was harmonized with those of SenderType and ReceiverType.
SimpleChannel: a simple-as-can be implementation of ChannelType, with (pretty much) no memory handling.

Can be used to puzzle over benchmarking issues. (And they are puzzling.)
chan-benchmark tweaks
documentation tweaks
Sender and Receiver, redone as structs.

With this, the performance of the no-contention message passing for the single-element buffered channel is down to about 165% of the equivalent operation in go (~260ns vs 160ns).

Message passing with contention is much slower than go, but the amount of time beyond the simple thread-swapping delay seems more than expected.