Provide compile-time access to CORE::X namespaces
Provide compile-time access to CORE::X namespaces

Make it possible to access classes of same name but defined in different
COREs: `CORE::C::PseudoStash` vs `CORE::E::PseudoStash`. Calling `.^ver`
on both would result in `6.c` for the former and `6.e` for the latter.

An example of use:

```
multi foo(CORE::C::PseudoStash:D $p) { ... }
multi foo(CORE::E::PseudoStash:D $p) { ... }
```

perl6/problem-solving#80

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