Add ModuleDataType to/from string serialization.

Add two new functions that leverage the RedisModuleDataType mechanism
for RDB serialization/deserialization and make it possible to use it
to/from arbitrary strings:

* RM_SaveDataTypeToString()
* RM_LoadDataTypeFromString()

tests/modules/Makefile

@@ -19,7 +19,8 @@ TEST_MODULES = \
     propagate.so \
     misc.so \
     hooks.so \
-    blockonkeys.so
+    blockonkeys.so \
+    datatype.so
 
 .PHONY: all