Turns out Container::call() does not work with invokable classes.
Thus, we need to wrap callables in a custom extender class to
support injecting any resolvable type-hint automatically.
Refs #851.
This simplifies the API and gives extension developers more
flexibility, for a) maintaining backwards compatibility, and
b) doing advanced stuff that extenders do not allow.
Note that only extenders are guaranteed to work across
different versions of Flarum (once the API surface is stable).
See the discussion in https://github.com/flarum/core/pull/1335.
Loading the activated extensions now means retrieving an array of
extenders (classes that implement a certain type of extension of a core
feature in Flarum).
For now, the only existing extender is the Compat extender which is used
to handle old-style bootstrappers that simply return a closure that
receives all of its dependencies via auto injection.
In the future, extensions will be able to return an array of extender
instances from their bootstrapper instead. These extender classes will
be implemented in the next step.
