add an implementation of std::variant<...>

This allows strengthening type safety in a number of data structures we
already have.

Author: Daniel Kroening

.clang-format-ignore

@@ -0,0 +1 @@
+src/nonstd/variant.h

src/nonstd/README.md

@@ -5,3 +5,8 @@
 
 `nonstd` contains implementations of C++ utilities that are not yet
 part of the standard library, e.g. for `optional`.
+
+variant.hpp implements `variant`; it is
+https://github.com/mpark/variant/blob/single-header/v1.4.0/variant.hpp
+License is at
+https://github.com/mpark/variant/blob/single-header/LICENSE.md