Fixing SetSlice, Reshape, TryCopyTo.

[michaelgsharp]

Fixes #106539, #106537, and #106535.

They all had to go in one PR as they were all connected, but basically fixes Reshape/SetSlice to work with 0 strides. Also adds a few more checks to make sure its ok to Reshape the values.

[soerenwolfers]

With this proposal, the definition of Reshape would be sufficiently restrictive that I think

• the documentation should be extended to specify in more detail what "the new shape is not compatible with the old shape" means
• the functionality should be hidden behind a TryReshape method.

Also, note that the bit where numpy and the current TensorSpan deviate in #106539 is just a.Reshape(a.Lengths), on which numpy returns a whereas the proposal here would throw an exception, which it should probably not (although maybe there is a reasonable definition of Reshape that doesn't allow it and I just don't see it, see the first bullet point).