RSTSR rust native implementation to simple operators

This crate includes some native implementation (such as tensor addition, reduction, layout-change operations).

Only CPU (both serial and rayon parallel) are of concern.

It is splitted from rstsr-core, so to make this crate rstsr-native-impl more emphasis on computation, and this rstsr-core more emphasis on device and tensor structure/trait definition.

This crate can also be used if you wish to use tensor computation utilities, without actually defining a tensor object. In another word, this crate is device-free. Only serial and rayon parallel is concerned in this crate.

THis crate uses utilities in rstsr-common, such as layout, broadcasting, iterator.