please add index by lua function

[UNera]

Now taratnool builds index by field value.

It would be nice to have indexes by lua function.

example:

function tlen(tuple)
  return #tuple
end

space[1].enabled = 1
space[1].index[0].type = "TREE"
space[1].index[0].unique = 0
space[1].index[0].index_function.name = tlen
space[1].index[0].index_function.type = NUM # return value

so tarantool should call 'tlen(tuple)' for each tuple while it is building index and while it is doing insert/update operation.

so using the index (in example) we can find largest tuples

[Roman Tsisyk (rtsisyk)]

=> system-spaces-v2