Avoid narrowing conversion in `optional`

This issue was causing compile warnings on Ubuntu 18, when compiling
with conversion warnings enabled. The result of `size_of` was being
converted to `unsigned` which is a narrowing conversion on 64 bit
platforms. This commit fixes this issue by changing the type to
`std::size_t`, which is large enough to store the resultof `size_of`
without a narrowing conversion.

src/nonstd/optional.hpp

@@ -356,7 +356,7 @@ struct alignment_of_hack
     alignment_of_hack();
 };
 
-template <unsigned A, unsigned S>
+template <std::size_t A, std::size_t S>
 struct alignment_logic
 {
     enum { value = A < S ? A : S };