Make stringable-object equivalent to Stringable

[weirdan]

Fixes #8575
Supersedes #8595

There were a number of problems with stringable-object (object{__tostring()}) type used by implode/join stubs:

1. stringable-object type is repesented internally as object{__tostring()} (lowercase), but checks in various places were using __toString casing (note the capital S).
2. There was no support for checking sub/supertypes involving stringable-object and Stringable interface.