Create an expr_kernel_generator base class for expr dtypes
Add a function which can make a struct of pointers to other ndobject data

Also tweak the fixedstruct constructor
Add struct and fixedstruct single comparison kernels
Add a groupby tests which uses a struct as the 'by' argument

Also a fix in struct assignment, was using the wrong
metadata in one place.
Start some utility functions for creating element-wise expr kernels