Add log macros as APIs

[tisonkun]

We'll still use log as the recommended log facade. But having Logforth's macros as APIs helps applications to leverage some Logforth-only features, like a more fine-grained log level.

Also, it helps us to evaluate whether Logforth's APIs are reasonable.

[pol-epsevg]

LGTM. Having a crate like logforth-macros could be useful. (It could be made available through the logforth crate by using a macros feature flag as is common in many crates)
Besides helping leverage Logforth-only features, it could be used to dispense with log entirely for some projects. If Logforth were to provide a facade, I'd prefer to use it instead of having to import log.
Also, log's macro expanision goes quite deep, so macros that are more tight-knit with Logforth could potentially cut down on compilation time too.