shallowSlice and shallowReshape are added. Tests pending.

shallowAxis(yields reference) and maxis (yields mutable reference, similar to items) are planned but will probably be blocked by nim-lang/Nim#5887 (shallowCopy in iterators yields full copies).

Discussion:

Add unsafeShallowSlice which can take "let" parameter (and not var) and writing fast libraries that uses Arraymancer. cc @edubart. Relevant discussion about shallow vs unsafeShallow in shallow copying does not seem to work nim-lang/Nim#644 and https://github.com/nim-lang/Nim/issues/3377
RFC in Nim bug tracker about slicing Views - https://github.com/nim-lang/Nim/issues/5753. Should we have a separate view type and have proc generics over Tensor and TensorView

My opinion:

It is important to provide sane defaults but allow higher performance when needed/possible.
unsafeShallow while long is good because easily grep-able and safe code is by default/easier to write ( '=' for copy, shallowSlice for var shallow, unsafeShallowSlice for let shallow).
Shallow copy by default is also a source of (bad) surprise for Numpy users.

Another explanation from the Matrix Template Library on the issue of shallowCopy for numerical software.
When Assignment overload does not distinguish between var, let, return values nim-lang/Nim#6348 is solved, Arraymancer will implement "move" optimization so that tensors with refcount of 1 will get shallow-copied on assignment using this.
Also it is possible to rely on Nim GC to efficiently get refcount with proc getRefcount.
Implementing Tensor and TensorView, CudaTensor and CudaTensorView, etc seems too cumbersome to use and maintain. Having shallow, unsafeShallow and getRefcount should already be explicit and flexible enough.

Uh oh!

Add slice, reshape, axis views for no-copy tensor manipulation #46

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