Move strbuf implementation to shared/

Just move the strbuf-like implementation to shared/. No renames were
made yet to avoid cluttering the diff. It will come in a separate patch.
diff --git a/Makefile.am b/Makefile.am
index 39b8e2e..e2c5301 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -50,6 +50,8 @@
 	shared/array.h \
 	shared/hash.c \
 	shared/hash.h \
+	shared/strbuf.c \
+	shared/strbuf.h \
 	shared/util.c \
 	shared/util.h