Fix accessors

[HugoGranstrom]

Right now there are a few tests in https://github.com/SciNim/flambeau/blob/master/tests/arraymancerTestSuite/tensor/test_accessors.nim that fail. Most notably:

• No bounds-checking
• Expression like a[1, 1] += 10 isn't possible because a[1, 1] is immutable.

[HugoGranstrom]

Added the slicer test as well: https://github.com/SciNim/flambeau/blob/master/tests/arraymancerTestSuite/tensor/test_accessors_slicer.nim And there are not many tests that work. Seems like the more advanced slices doesn't work really like _ and ^.