Add initial code for suspending execution

This is just some initial code, which is still quite broken (and needs to be
moved so it can be reused.)

Add some boilerplate code for Generator class

The Generator class now uses a zend_generator struct, so it'll be able to
store additional info.

This commit also ensures that Gene

Add some boilerplate code for Generator class

The Generator class now uses a zend_generator struct, so it'll be able to
store additional info.

This commit also ensures that Generator cannot be directly instantiated
and extended. The error tests are now in a separate folder from the
(yet-to-come) functional tests.

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Add empty Generator class