Include needed headers for concepts and std::errc

Merge request feedback pointed out missing headers, so I added them. Also
changed to using the std::same_as concept instead of the type_traits version
since we are already using concepts anyways.

src/snitch_append.cpp

@@ -1,7 +1,11 @@
 #include "snitch/snitch_append.hpp"
+#include "snitch/snitch_string.hpp"
 
 #include <charconv> // for std::to_chars
 #include <cstring> // for std::memmove
+#include <concepts> // for std::floating_point, std::same_as
+#include <system_error> // for std::errc
+
 
 namespace snitch::impl {
 namespace {
@@ -17,7 +21,7 @@ using snitch::small_string_span;
 template<std::floating_point T>
 bool append_to_chars(small_string_span ss, T value) noexcept {
     constexpr auto fmt       = std::chars_format::scientific;
-    constexpr auto precision = std::is_same_v<float, T> ? 6 : 15;
+    constexpr auto precision = std::same_as<float, T> ? 6 : 15;
     auto [end, err] = std::to_chars(ss.end(), ss.begin() + ss.capacity(), value, fmt, precision);
     if (err != std::errc{}) {
         // Not enough space, try into a temporary string that *should* be big enough,