Add type restrictions to Log directory

[Vici37]

This is the output of compiling cr-source-typer and running it with the below incantation:

CRYSTAL_PATH="./src:lib" ./typify spec/all_spec.cr 
--error-trace 
--exclude src/crystal/ 
--stats --progress 
--ignore-private-defs 
--ignore-protected-defs 
--union-size-threshold 2 
--include-splats 
src/log

This is related to #15682 .

[Sija]

.new methods are always returning self, making return type annotation IMO pretty redundant.

[ysbaddaden]

For documentation purpose, yes, but there are cases where the compiler doesn't always infer that a specific .new returns a self.

[nobodywasishere]

I believe we're on the side of preferring no type restriction instead of _ or forall if the type can't be restricted. See #15334

[straight-shoota]

If the type is really unrestricted - like in this case - I think it makes a lot of sense to explicate that in the type restriction _.
Otherwise, it would be indiscernible from an unknown type restriction.

[nobodywasishere]

That is also my preference, especially from a documentation perspective, but I know others on the team disagreed.

Another alternative proposed in that issue:

def context(*, before : B = nil, after : A = nil) : Nil forall A, B
end

[straight-shoota]

_ is more concise than forall, so I'd use that.