Glue to hslua for aeson values.

This provides a StackValue instance for aeson's Value type. The following conventions are used:

• Null values are encoded as a special value (stored in the registry field HSLUA_AESON_NULL). Using nil would cause problems with null-containing arrays.
• Objects are converted to tables in a straight-forward way.
• Arrays are converted to lua tables. Array-length is included as the value at index 0. This makes it possible to distinguish between empty arrays and empty objects.
• JSON numbers are converted to Lua numbers (usually doubles), which can cause a loss of precision.