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/meson.build b/src/meson.build
index b367dc4..ab786c0 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -50,6 +50,7 @@
   'hash.cpp',
   'hash/array.cpp',
   'hash/tuple.cpp',
+  'numeric/endian.cpp',
   'pinned.cpp',
   'raw.cpp',
   'signal.cpp',