[SPARK-9404][SPARK-9542][SQL] unsafe array data and map data

This PR adds a UnsafeArrayData, current we encode it in this way:

first 4 bytes is the # elements
then each 4 byte is the start offset of the element, unless it is negative, in which case the element is null.
followed by the elements themselves

an example:  [10, 11, 12, 13, null, 14] will be encoded as:
5, 28, 32, 36, 40, -44, 44, 10, 11, 12, 13, 14

Note that, when we read a UnsafeArrayData from bytes, we can read the first 4 bytes as numElements and take the rest(first 4 bytes skipped) as value region.

unsafe map data just use 2 unsafe array data, first 4 bytes is # of elements, second 4 bytes is numBytes of key array, the follows key array data and value array data.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7752 from cloud-fan/unsafe-array and squashes the following commits:

3269bd7 [Wenchen Fan] fix a bug
6445289 [Wenchen Fan] add unit tests
49adf26 [Wenchen Fan] add unsafe map
20d1039 [Wenchen Fan] add comments and unsafe converter
821b8db [Wenchen Fan] add unsafe array