Merge pull request 'Reintroduced previously erroneously removed memcpy' (#5) from fix/memcpy into master

Reviewed-on: http://git.mercurial-manifold.eu/an.tsouchlos/const_fmt/pulls/5

const_fmt/format_impl.h

@@ -95,6 +95,10 @@ constexpr inline const char* digits2_base(size_t value) {
 
 
 constexpr inline void copy2(char* dst, const char* src) {
+    if (!std::is_constant_evaluated()) {
+        std::memcpy(dst, src, 2);
+        return;
+    }
     *dst++ = static_cast<char>(*src++);
     *dst   = static_cast<char>(*src);
 }