channels: Use context in all functions

[orsinium]

This is a big rewrite of almost all functions in the channels package. It introduces a new function with C suffix for every function. For example, First and FirstC. This new function accepts a context as the first argument and respects this context in all blocking operations.

Also:

1. Add golangci-lint config.
2. Rewrite documentation. The module-level documentation for channels now covers all you need to know.
3. Add some new functions for channels. EchoC for sure is new, maybe some others too. I didn't keep track >.<