Remove multiple definition of `core_debug'.

The function `core_debug' was implemented in three different places, each time
with a slighly different definition.

The only definition we keep is the one in Dancer2, and we don't need to export
it in the DSL, since this sould be used *only* by Dancer2.
Merge branch 'devel' into fix/remove-multi-core-debug
Merge branch 'fix/remove-multi-core-debug' into devel
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