`double` field type new behaviour

[TarantoolBot]

Double field type now is identical to number. I.e. it can
store integers and compares them as integers.

• Reference -> SQL reference -> SQL user guide:
  DOUBLE values are numerics that do contain decimal points (for
  example 0.5) or are expressed with exponential notation (for
  example 5E-1) -> now DOUBLE can have any numbers, including
  integers. It becomes identical to NUMBER.

• Platform -> Defining and manipulating data -> Data storage:
  the storage type is MP_DOUBLE and the size of the encoded
  value is always 9 bytes. In Lua, fields of the double type
  can only contain non-integer numeric values and cdata values
  with double floating-point numbers -> not so anymore.
  double field is now identical to number (it can store
  integers too, and compares them like integers).
  Requested by @Gerold103 in tarantool/tarantool@60272e8.