Add Skip input to Fold Step

[harendra-kumar]

A Fold can say to the driver that "call me without input the next time". This is symmetric to the Stream Step skipping the output using the Skip Step constructor. This change will make the Fold Step completely symmetric to the stream Step. Like the stream Skip allows filtering output and chunked folding of stream, the same way the Skip input in folds would allows for filtering input and unfolding of chunks in the input of a fold.

This will solve the performance issue in modular unicode encoding.

[harendra-kumar]

Note: In Parsers a workaround for Skip is to use Partial 1.