Name context functions consistently

[krnowak]

It would be nice to follow a single schema for naming context
functions. In the trace package we followed the form FooFromContext
and ContextWithFoo. Do the same in the correlation package. The schema
WithFoo is mainly used for functions following the options pattern.

Not sure about a name of the NewContext function, though. For now I
have left it alone.

[jmacd]

I support consistency. :-)

For the record: If you asked what I prefer, I think I'd prefer to see functions like CurrentMap(ctx) as opposed to MapFromContext(ctx), CurrentSpan(ctx) as opposed to SpanFromContext(ctx), ...

[krnowak]

I support consistency. :-)

For the record: If you asked what I prefer, I think I'd prefer to see functions like CurrentMap(ctx) as opposed to MapFromContext(ctx), CurrentSpan(ctx) as opposed to SpanFromContext(ctx), ...

And CurrentRemoteSpanContext instead of RemoteSpanContextFromContext. Sounds ok as well to me. I'm fine with either. So getters could follow the CurrentFoo scheme. What about setters? They currently follow the ContextWithFoo scheme. Maybe SetCurrentFoo?

[krnowak]

Heh, see #185 and #379. :)