Add zip_map adaptor

[isaacy2012]

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Addresses, #172: The element type of flux::cartesian_product, cartesian_power and adjacent is a tuple of the element type(s) of the underlying sequences. These three adaptors also provide *_map versions, which take an n-ary function and call it directly with the underlying elements. This is to avoid forming a tuple and then immediately destructuring it, as would happen in a subsequent call to map(unpack(func)).

This commit adds the *_map version for the zip adaptor.