Implement "popular" top-level functions of numpy

[dmoisset]

Many users use for example numpy.dot(a, b) instead of a.dot(b). Having the top level versions of those functions should cover more use cases

[kjyv]

Apart from global variants of array instance methods there are also others like load or save which don't exist as instance method (save not for multiple arrays), so it's actually more than a nice to have.