Improve the `parsers` API

Since this was merged: https://github.com/scrapinghub/dateparser/pull/603, the "parsers" API has been improving.

I think that we should define better what's a parser and define the same API for all of them, refactoring the code.

This would allow a clearer code, to ensure that all the rules (settings, etc) are applied in all the parsers, and it would also allow building some new fancy public functions to use the parsers directly like: parse_timestamp() or parse_absolute_time().

For that, we would move parts of the _DateLocaleParser logic inside each parser.

My idea is:

A "parser" is a function that receives a date_string and the settings and it returns the parsed datetime, None, or raises an error.