fixes for gcc11

Gerhard Ravenrequested to merge
fix-issue-150 into master
  • fixes #150 (closed): tweak meta_enum to be constexpr with gcc11
  • add missing include
  • make sure zip iterators really satisfy the random access iterator concept. Specifically, for an iterator iter, the offset dereference operator, iter[n] should exist, and be equal to *(iter+n). See here.
Edited by Gerhard Raven

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