Invariants over states in WP and get rid of heap_ctx.
The WP construction now takes an invariant on states as a parameter (part of the irisG class) and no longer builds in the authoritative ownership of the entire state. When instantiating WP with a concrete language on can choose its state invariant. For example, for heap_lang we directly use `auth (gmap loc (frac * dec_agree val))`, and avoid the indirection through invariants entirely. As a result, we no longer have to carry `heap_ctx` around.
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