Add zip_map adaptor

[isaacy2012]

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.

[tcbrindle]

So first of all, please accept my apologies @isaacy2012 for letting this PR languish for months. I'm afraid I was pretty busy at the time you originally submitted it so I intended to review it later, and then I somehow I never got back to looking at it. I feel very bad!

The good news is that this PR looks great! I've just got a couple of small comments but otherwise this looks awesome.

If you've moved on in the intervening time and don't feel like coming back to this then I quite understand :). I'll be happy to do a final polish if you're busy with other things.

Thanks, and apologies again

[isaacy2012]

Hi @tcbrindle, no worries at all, I'll address those changes this weekend :)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.22%. Comparing base (d7bbf7c) to head (6cefba1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #184 +/- ## ======================================= Coverage 98.22% 98.22% ======================================= Files 69 69 Lines 2360 2360 ======================================= Hits 2318 2318 Misses 42 42 ```

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[tcbrindle]

Thanks @isaacy2012!