numeric/str: Add constexpr int encode

This makes it possible to quickly convert numbers to strings in
constexpr contexts.

Change-Id: I5a460feacf97c80761d02a3b07f5b22080401b45
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/meson.build b/src/meson.build
index 93d68f3..6d17d3d 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -54,6 +54,7 @@
   'net/addr/ip.cpp',
   'net/addr/subnet.cpp',
   'numeric/endian.cpp',
+  'numeric/str.cpp',
   'pinned.cpp',
   'raw.cpp',
   'signal.cpp',