Improve some of the Ravi extension APIs

[dibyendumajumdar]

ravi_typename() should call metamethod __name on userdata or table types
Add raviL_tolstring() auxiliary library function - this should use the extended ravi typenames
Modify the Ravi metatable functions to take a type name and set _name attribute on metatable
Rename the Ravi metatable functions to start with raviL.
Add an API function to get access to the integer data array of a integer[] variable.
Modify the API functions for getting access to raw data so that they also return the array length

[dibyendumajumdar]

Resolved in 0.17