Closure::bind() and Closure::bindTo() are currently reported as aliases in stubs because they have a single implementation. They are not aliases in fact though, they just use zend_parse_method_parameters() cleverly.
Thus, let's separate their implementation so that we don't have to alias Closure::bindTo() anymore. This will also have the advantage that the two ZPP implementations become more clear.
Closes GH-6169
Refactor the zend_is_callable implementation to check callability
at a particular frame (this is an implementation detail for now,
but could be exposed in the API if useful). Pick the first parent
user frame as the one to check.
We also change `Generator::throw()` to expect a `Throwable` in the
first place, and we now throw a TypeError instead of returning `false`
from `Exception::getTraceAsString()`.
