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/numeric/str.cpp b/src/numeric/str.cpp
new file mode 100644
index 0000000..7c3ac13
--- /dev/null
+++ b/src/numeric/str.cpp
@@ -0,0 +1,7 @@
+#include <stdplus/numeric/str.hpp>
+
+namespace stdplus::detail
+{
+template char* uintToStr<16>(char*, uintptr_t, uint8_t) noexcept;
+template char* uintToStr<10>(char*, uintptr_t, uint8_t) noexcept;
+} // namespace stdplus::detail