std.log: add scoped logging struct
std.log: add scoped logging struct

 * Add a std.log.scoped function that returns a scoped logging struct
 * Add a std.log.default struct that logs using the .default scope

Implementation of daurnimator's proposal:
https://github.com/ziglang/zig/issues/5943#issuecomment-669043489

Note that I named the function "scoped" instead of "scope" so as not to
clash with the scope parameter that is used everywhere; this seemed a
better solution to me than renaming the scope parameter to "s" or
"log_scope" or the like.

zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig