Auto merge of #395 - Xanewok:heap-boxed, r=jdm
Add safe Heap::boxed constructor
While this doesn't remove old Heap constructor (#343), it adds a new `boxed` function along with some additional explanation while this may be preferred until a better/different solution comes up to construct a Heap value directly with an appropriate value set, in a safe manner.
This made my work (which I'll upload later) on typed arrays, and also indirectly for objects, in webidl unions safer and more convenient, so I think it's a good addition.
r? @jdm
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