Specifiy type of set in toTensor for HashSet | OrderedSet (#656)

* [tensor] avoid usage of `SomeSet` in `toTensor` for sets

The generic + generic type class leads the compiler to become drunk
and chooses to compile code that really does not belong in there.

We might further want to restrict `T` as well, but I don't want to be
too restrictive for now.

* [tensor] reorder the `sets` `toTensor` to use `len` of produced seq

src/arraymancer/laser/tensor/initialization.nim

@@ -276,19 +276,19 @@ proc toTensor*[T; U](a: openArray[T], typ: typedesc[U]): Tensor[U] {.inline.} =
   else:
     toTensor(a).asType(typ)
 
-proc toTensor*[T](a: SomeSet[T]): auto =
+proc toTensor*[T](a: HashSet[T] | OrderedSet[T]): Tensor[T] =
   ## Convert a HashSet or an OrderedSet into a Tensor
   ##
   ## Input:
   ##      - An HashSet or an OrderedSet
   ## Result:
   ##      - A Tensor of the same shape
   var shape = MetaData()
-  shape.add(a.len)
   let data = toSeq(a)
+  shape.add(data.len)
   result = toTensor(data, shape)
 
-proc toTensor*[T; U](a: SomeSet[T], typ: typedesc[U]): Tensor[U] {.inline.} =
+proc toTensor*[T; U](a: HashSet[T] | OrderedSet[T], typ: typedesc[U]): Tensor[U] {.inline.} =
   ## Convert a HashSet or an OrderedSet into a Tensor of type `typ`
   ##
   ## This is a convenience function which given an input `a` is equivalent to