Add Slice tensor operation.

[nkoppel]

Resolves #575 and #591.

Todo:

• More tests
• Documentation
• More dtypes for slice and reshape (reshape does not work on tensors of type usize #591)

[nkoppel]

My current implementation of adding dtypes to slice and reshape doesn't implement backward functions for the integer dtypes, so these will error at runtime if the backward operations are called for these dtypes on cuda.

[coreylowman]

Awesome, nice work!