gorgonia/tensor: Some performance updates and bugfixes

v0.9.6 saw some updates to the Repeat operation. This release fixes some performance hits that the Repeat operation had, by adding fast path updates to special, common cases. Furthermore some bugs were fixed iterator use check- where a check used to be made to see if there both a and b have the same data order (i.e. C-ordered tensors can interact nicely with other C-ordered tensors without needing iterators, but if it interacts with a Fortran-ordered tensor, then an iterator is required). This check was wrong, and has been fixed. a potential unsafe slice casting was also fixed by @jlauinger . Previously if a GC run is called in the middle of a AsByteSlices(), then the allocated []byte would have been collected before it returned. This has been fixed