Auto merge of #479 - romankl:r-change-slot-count, r=jdm
change the slot count to match the spidermonkey defintion
right now the slot count is `2 + 38` in the spider monkey fork. this PR changes it to match the C++ definition.
The definition in C++: https://github.com/servo/mozjs/blob/dd175ee73a179ce218286c4be4ab80e1a75348be/mozjs/js/public/Class.h#L841
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