Type check accessors [], []= and add index_select for multiple values

[Vindaar]

This finally makes the [], []= mostly "type safe". Just handing in any type and falling back to the compile time errors of the (mostly internal) procedures like index_select is a thing of the past now. If the argument is not allowed, we simply throw a (better) error message telling the user that the type is not valid.

On Nim devel we can also type check Tensor[T] (i.e. the inner type), because nowadays doing

  let validTensorTypes = [getTypeInst(Tensor[int])[1],
                          getTypeInst(Tensor[bool])[1]]
  for t in validTensorTypes:
    if sameType(typ, t): return true

works as expected. Nim issue #14021 nim-lang/Nim#14021 should in principle be fixed on devel (but not 2.0.4). Haven't check the Nim issue fully yet though.

This also adds support to not only index a tensor at multiple indices, but also assign multiple at the same time:

z2[[0, 2]] = [5.2, 1.2]
z2[[0, 2]] = @[5.2, 1.2]
z2[@[0, 2]] = @[5.2, 1.2]
z2[@[0, 2]] = toTensor [5.2, 1.2]
z2[toTensor [0, 2]] = [5.2, 1.2]
z2[toTensor [0, 2]] = @[5.2, 1.2]
z2[toTensor [0, 2]] = toTensor [5.2, 1.2]

are all valid now. Previously we could only read with multiple indices.

Note that multi dimensional slicing of arbitrary indices is still not supported.

[AngelEzquerra]

This is great, @Vindaar. Other than the weird git formatting on 3211c4e3211c4e, that we already discussed offline these changes (and this functionality!) look good to me!