Adding support for getting/setting tensor slices

[seanlatias]

We should add support for the following features.

1. Get a slice of a tensor and make a copy

   A = hcl.placeholder((10,))
   B = A[2:5].copy()
   # equivalent API
   B = hcl.compute((3,), lambda x: A[x+2])

2. Set a slice of a tensor

   A = hcl.placeholder((10,))
   A[2:5] = [1, 2, 3]
   A[2:5] = 4  # broadcast

3. Get a slice of a tensor without making a copy

   A = hcl.placeholder((10, 10))
   B = A[2:5]
   B[0] = 1
   hcl.print(A[2]) # returns 1