Add matrix Redis datatype

* Adds XGET and XSET commands
* XSET rplies with dimensions and shape of matrix
* XGET replies with content of selected subregion of matrix

Shape reply:
* Number of dimensions (dims)
* dims replies for the size of each dimension (shape)

Content reply:
* Number of dimensions (dims)
* dims replies for the size of each dimension (shape)
* product(shape) doubles for each value
Add XZEROS and XONES for creating matrices
Add XEYE command for creating identity matrices
Add rdb persistence for matrix type