Add isFinite, first & last to rrule & rruleset

[ephys]

This implements feature request https://github.com/jakubroztocil/rrule/issues/215

• isFinite returns true if the rrule/rruleset has a determinable final occurrence
• last returns the final occurrence if it exists and is determinable
• first returns the first occurrence if it exists

This PR also implements the following revertible changes:

• calling count on an infinite rrule/rruleset now returns Number.POSITIVE_INFINITY instead of returning all possible dates until the year 9999 - could be considered breaking.
• calling all on an infinite rrule/rruleset now throws instead of returning all dates until the year 9999 (unless an iterator has been provided as the iterator can stop the loop) - could be considered breaking.
• options.count throws if it received something other than a safe integer, null or Number.POSITIVE_INFINITY. Number.POSITIVE_INFINITY normalizes to null.

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[KamalAman]

This would be a lovely PR to add if it is still valid.

[dereekb]

Was just looking to add this functionality to my project, specifically checking for finiteness and finding the last date.