Remove `Open Scope Z_scope` in HeapLang.

tests/heap_lang.ref

tests/heap_lang.v

tests/ipm_paper.v

tests/tree_sum.v

theories/heap_lang/derived_laws.v

theories/heap_lang/lang.v

theories/heap_lang/lib/array.v

theories/heap_lang/lib/counter.v

theories/heap_lang/lib/ticket_lock.v

theories/heap_lang/metatheory.v

theories/heap_lang/primitive_laws.v

theories/heap_lang/proofmode.v

theories/heap_lang/proph_erasure.v