Move some of Sink/Source member functions to extensions

[fzhinkin]

Sink/Source interfaces declare several member functions that could be easily implemented as extension functions. For example, Source::peek, Source::transferTo, and Sink::transferFrom.

Having such functions implemented as member functions does not provide any obvious benefits but requires us to write similar implementations several times. It would be nice to have a single implementation instead.

[joffrey-bion]

Having such functions implemented as member functions does not provide any obvious benefits

It allows for different implementations to optimize these methods, specifically by offering the option to implement them differently. If those are moved to extension functions, then we face the same hell as with extensions on collections which cannot be easily optimized for specific collection implementations.

Typically, you faced it yourself already: #342, #343

but requires us to write similar implementations several times

I may be missing something, but why not just have a default implementation in the interface itself?

[fzhinkin]

I may be missing something, but why not just have a default implementation in the interface itself?

That's definitely an option to consider