Support for generics.

[allComputableThings]

Feature request

Currently, I don't think its possible to have a template jitclass. For example, in the

https://github.com/numba/numba/blob/master/examples/stack.py

... the Stack class can only be instantiated to a single element type (in the example, an int).

It would be nice to be able to re-use the stack class for multiple element types:

Something like this:

  IntStackClass = Stack(elemtype=int) # A template class is function (type params ...)->class
  IntStackClass = Stack.instantiate(elemtype=int)
  IntStackClass = numba.instantiate_types(StackTemplate, elemtype=int )
  ...
  IntStackClass().push(1)

We can already do this by wrapping the creation of the stack class in a function who's parameter is elemtype. However, this isn't particularly nice since:

• it makes the Stack class undocumented when its defined within a function
• is rather inefficient since we're creating a Stack class object over and over for each call and additional memoization is required to prevent this,

Perhaps:

  @jitclass_template(
      template_type=("elemtype"),
      spec=( ("head": deferred_type()),
                  ("size": numba.intp),
  )
  class Stack(object)
      def __init__():
          self.head = None
          self.size=0

     def push():   
          if self.head is None:
                 self.head = LinkedNode(Stack.elemtype)()  # Template instantiation reduce to LinkedNode#<elemtype>()

          ...