Declare tentative return types for ext/intl (#6986)

Generate ext/intl class entries from stubs

Closes GH-6670

Improve parameter names in ext/intl

Closes GH-6309

Warning to ValueError promotion in Intl extension Part 1

Affects:
- IntlCalendar
- IntlGregorianCalendar
- IntlBreakIterator

Closes GH-5669

Introduce InternalIterator

Userland classes that implement Traversable must do so either
through Iterator or IteratorAggregate. The same requirement does
not exist for internal class

Introduce InternalIterator

Userland classes that implement Traversable must do so either
through Iterator or IteratorAggregate. The same requirement does
not exist for internal classes: They can implement the internal
get_iterator mechanism, without exposing either the Iterator or
IteratorAggregate APIs. This makes them usable in get_iterator(),
but incompatible with any Iterator based APIs.

A lot of internal classes do this, because exposing the userland
APIs is simply a lot of work. This patch alleviates this issue by
providing a generic InternalIterator class, which acts as an
adapater between get_iterator and Iterator, and can be easily
used by many internal classes. At the same time, we extend the
requirement that Traversable implies Iterator or IteratorAggregate
to internal classes as well.

Closes GH-5216.

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Generate method entries for ext/intl

Closes GH-5370

Add stubs for Intl BreakIterator

Closes GH-5207