str/conv: Add basic functions

We need some basic templates for formatting into a buffer or fmtlib to
make other conversions straightforward.

Change-Id: I8c13175394f6b4fd4a55edf8d653e98a1562cc64
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/str/conv.cpp b/src/str/conv.cpp
new file mode 100644
index 0000000..22897a0
--- /dev/null
+++ b/src/str/conv.cpp
@@ -0,0 +1 @@
+#include <stdplus/str/conv.hpp>