composed functions can optionally supply namespace

[jcmgray]

When you write a composed function, the first and default version can now take a namespace parameter (it could already accept a backend parameter):

@compose
def my_func(x, namespace):
    # get how many elements are needed to sum to 20
    return namespace.sum(namespace.cumsum(x, 0) < 20)

which is convenient and also saves dispatching overhead.