ZIO-2 logging (Part 1)

[vagroz]

This PR copy-pastes ZIO-1 implementation with small improvements.
The docs and ZIO-Native implementation will be brought with another PR.

[FunFunFine]

Is there any other way to get implement this method other than that? It feels like any implementation of ContextProvider will be doing something.something.map(x => /*implicit conversion*/ x). Perhaps it's better to make it ContextProvider[A: Loggable]?

[vagroz]

ZLogs.layerPlainWithContext is pretty now: we can provide to it any implementation of ContextProvider.
If we make this trait as ContextProvider[A: Loggable], the layer will also require type-parameter which can be huge enough. For example, next PR will bring ContextProvider with data A = Map[ZLogAnnotation[_], Any].

As a user, I don't want write THIS type A, ideally I should not know anything about it. I just write two things

zioProgram.provide(
  ZLogs.layerPlainWithContext,  // dear tofu, give me a logger factory using ContextProvider I point next to
  TofuDefaultContext.layer         // I'd like to log this context, don't carry how this collected
)

[vagroz]

will be doing something.something.map(x => /implicit conversion/ x)

suggest abstract class ValueContextProvider which do this itself

[FunFunFine]

suggest abstract class ValueContextProvider which do this itself

yep, that'll work