feat: import Collections library

[alex-chew]

This PR imports the Collections libraries from dafny-lang/libraries, as a new DafnyStdLibs library.

Note that the LittleEndianNat and LittleEndianNatConversions libraries are not imported, as there is still work required to enable different verification flags in the standard libraries (particularly --disable-nonlinear-arithmetic as desired for these libraries).

Description

How has this been tested?

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[robin-aws]

I think this is good to merge once the dependency Arithmetic is in, especially to unblock other libraries that depend on it. There are a couple of things to follow up on afterwards though:

• Obviously needs examples. Using feat: Execution branch coverage report #4755 to measure coverage will point out that they are missing so I'm not worried about forgetting about this.
• Some functions are ripe for performance improvements - probably quite a few need {:tailrecursion} to avoid stack overflows on large inputs, and perhaps some could use by method optimizations. I wouldn't block on that in general, but we did do a little bit of this to ensure this JSON stress test passes: https://github.com/dafny-lang/libraries/blob/master/src/JSON/Tests.dfy#L134-L136 If that regresses it means we've somehow lost an optimized version in transition.

[robin-aws]

Changed my mind on my previous review - didn't realize that Math had definitions that aren't already in Arithmetic. The distinction is not common in standard libraries, but I think Dafny's allowed to be more precise than usual. :)