numeric/endian: Add constexpr endian conversion functions

These are as fast as the c library versions but they are also constexpr.

Change-Id: I95bb1562a5a15a7f9c57a3483afe364a0286da48
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/numeric/endian.cpp b/src/numeric/endian.cpp
new file mode 100644
index 0000000..97600cf
--- /dev/null
+++ b/src/numeric/endian.cpp
@@ -0,0 +1 @@
+#include <stdplus/numeric/endian.hpp>