Restore Heap example

[aspiwack]

Currently deactivated because of a linearity bug. (I suspect it's due to the fact that workers for a binary data constructor currently uses the same multiplicity for both fields, hence an issue in the linear ghc branch).

[aspiwack]

In fact the problem looks quite a bit more subtle. The issue doesn't manifest when there is a single alternative [ https://github.com/tweag/linear-base/blob/c3b1f2b89c67098269c861c2bcc2fadc9e741480/examples/Foreign/Heap.hs#L49-L51 ]. This is a subtle bug, but I'm sure that we will encounter it again.

For the moment, I'll leave this test deactivated.