Feature/variant_collection (#26)

* added variant_collection

* added variant_collection

* avoided &* on null pointers

* made (non-public) fixed_variant ctor explicit

* tested higher-arity visit

* implemented visit<void>

* fixed {boost::variant2|std}::variant insertion

* fixed lookup issues with invoke_visit

* removed unneeded constexpr qualifiers

* s/typeid_/index

* reverted c6bc62f6d2 as Clang 5.0 didnt seem to like it

* reinstated c6bc62f6d2

* dropped -std=c++1z for Clang 5.0

* updated docs and examples

* added boost::poly_collection::visit_by_index

* typo

* explicit cted tuple in make_iota_tuple

* changed function name to see if it helps with mangling-related Clang 3.8 ICE

* rewritten make_iota_tuple to try to make Clang 3.8 happier

* added boost::variant_collection_of

.github/workflows/ci.yml

@@ -90,7 +90,7 @@ jobs:
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
-            cxxstd: "11,14,1z"
+            cxxstd: "11,14"
             os: ubuntu-latest
             container: ubuntu:18.04
             install: clang-5.0