YAML 1.2 Core Schema support

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... or how to get rid of the no(rway) problem

Issue to track necessary changes for supporting the YAML 1.2 Core schema.

Suggested steps

• Add a test for YAML 1.1 types that can be easily extended for 1.2 testing Add a test for YAML 1.1 types #483
• Move around methods from SafeLoader to BaseLoader, and add methods to load a whole group of tags, so that users can choose tags in a more modular way
• Add methods for loading 1.2 Core tags. Since 1.1 and 1.2 share some tags, the previous step will be helpful to save some code
  (The same has to be done for the Dumper/Representer of course)
  Support for the YAML 1.2 Core and JSON schemas [Take 2] #555

Users will then be able to explicitly choose between the classic, default 1.1 SafeLoader and the Core Loader.
Ideally things like merge keys and other tags (that are now on by default and can't be turned off) can be chosen optionally.

Related issues

The following issues/PRs are more or less related to 1.2, and some can simply be fixed when the users have the possibility to choose 1.2 Core instead of 1.1.

#473
#414
#247
#173
#168
#116
#98
#89
#53
#19
#174
#248
#577

[perlpunk]

#512 adds CoreLoader/CoreDumper, JSONLoader/JSONDumper