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/test/meson.build b/test/meson.build
index 1e15a0e..51bc288 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -6,6 +6,7 @@
   'hash': [stdplus_dep, gtest_main_dep],
   'hash/array': [stdplus_dep, gtest_main_dep],
   'hash/tuple': [stdplus_dep, gtest_main_dep],
+  'numeric/endian': [stdplus_dep, gtest_main_dep],
   'pinned': [stdplus_dep, gtest_main_dep],
   'raw': [stdplus_dep, gmock_dep, gtest_main_dep],
   'signal': [stdplus_dep, gtest_main_dep],