Core.pm: use strict; use warnings;

Core::DSL does not need to use Dancer2::Core either.
Reorganize and update documentation for templates modules.
Unify the use of croak to verify if the template is a file or ref.
Merge branch 'doc/template' into devel

* doc/template:
  Unify the use of croak to verify if the template is a file or ref.
  Reorganize and update documentation for templates modules.
Merge branch 'Pr/cleanup_core_methods' of git://github.com/veryrusty/Dancer2 into veryrusty/cleanup_core_methods

* 'Pr/cleanup_core_methods' of git://github.com/veryrusty/Dancer2:
  Core.pm: use strict; use warnings;
Merge branch 'veryrusty/cleanup_core_methods' into devel

* veryrusty/cleanup_core_methods:
  Core.pm: use strict; use warnings;