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/meson.build b/src/meson.build
index a064f14..93d68f3 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -60,6 +60,7 @@
   'str/buf.cpp',
   'str/cat.cpp',
   'str/cexpr.cpp',
+  'str/conv.cpp',
   'str/maps.cpp',
   'util/cexec.cpp',
   'variant.cpp',