Add DIM operator and operations on shape_t values

Shape inference derives the shape of tensors in
the graph from input shapes. Operations such as RESHAPE
may need calculations to derive the new tensor shape.
This patch:

- Adds a DIM operator to get the size of a tensor in
  a given axis as a rank 0 tensor of type shape_t

- Allows RESHAPE to take a 1D shape tensor as input for
  the new shape

- Allows RESIZE, TILE, PAD to take input sizes based
  on shape tensors.

- Allows ADD, SUB, MUL, INTDIV to operate on rank 0
  shape_t tensors

- Allows CONCAT to concatenate 0D shape_t tensors to
  a 1D shape_t tensor

- Adds CONST support for shape_t tensors

In this version of the specification shape tensors must
be resolvable to constants at backend compile time.

Signed-off-by: Dominic Symes <dominic.symes@arm.com>
Change-Id: I484bd44452453b5e05d0d8a82689564587b224e4