Rename IsStream.fold to IsStream.runFold

[adithyaov]

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See #1016 for some context.

Having both Fold.fold and IsStream.fold might be confusing to the users. The best name that we currently came up with is IsStream.runFold. Suggestions are welcome.

We need to keep in mind that additional changes need to be made for consistency. If we rename IsStream.fold to IsStream.runFold, we'll also need to rename,

• unfold to runUnfold
• pipe to runPipe
• parse to runParse etc.

What about foldMany and parseMany? We have some kind of an implicit relation between fold and foldMany. In fact, fold was once named foldOnce.

Should they be renamed to runFoldMany and runParseMany?