pivot function implementation

[aregm]

[dchigarev]

Current state:
pivot is one of the reshape methods that have design documentation. However, implementation with the following of design caused unexpected performance issue that explained in details in this comment, in a few words - it's because of extremely large input of reduce function at the second phase of groupby_reduce.

There is another implementation, that uses draft implementation of unstack from #1649, which performs significantly better. For now which implementation to use is under discussion. Both implementation could be found at #1645.