Simplify internal logger resolution

[medikoo]

While there's nothing much sophisticated in logger resolution logic, it's kinda magical and internally requires changes to object prototypes. That is not typical, and makes implementation more convulted and not that portable to ES3 enviroments (altough it's not clear yet, whether there's value in doing that).

Update would make API usage more restrictive and also more straightforward:

• info logs will have to be logged via log.info() (log will no longer be a function)
• Access of other logger functions via logger function will not be possible (e.g.

log.debug.error('some error log');

will crash with undefined is not a function

• Still we may consider to maintain a possibility to preload loggers as:

const log = log.debug; log('some debug log')`