Rename core::info, etc. and modernize

[brson]

Constants named info, etc. are imported by default, and they can't be shadowed by variable bindings. At the minimum they should have less intrusive names, and follow the appropriate naming convention for constants.

They probably deserve to be enums, and therefore lang items.

These are rarely used, most people favoring the macros, so I think they should not even be imported by default, moved to core::logging.

[brson]

Logging enums won't need to be lang items once log is a macro.

[brson]

I think we should just get rid of these and log. Everybody uses the macros.