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/include/meson.build b/include/meson.build
index 5785caf..1a1eb6f 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -9,6 +9,7 @@
   'stdplus/hash.hpp',
   'stdplus/hash/array.hpp',
   'stdplus/hash/tuple.hpp',
+  'stdplus/numeric/endian.hpp',
   'stdplus/pinned.hpp',
   'stdplus/raw.hpp',
   'stdplus/signal.hpp',