fix: modify 'shape' access + memoize 'len'

[xxxxxxxxxmr]

Pretty straightforward, using the shape already on the object for unravel_index and ravel_index (self.shape instead of taking a shape parameter)

[xxxxxxxxxmr]

ca004b5

• added a 'usize' field (allows to continue deriving Clone and Copy).
• the len method now simply returns the value stored in the len field.
• updated the constructor methods (zeros, ones, and new) to calculate the length and store it in the len field when creating a new Dimensional instance.