Add `Array.Swap()` to swap two elements in place

[fxamacker]

Updates onflow/cadence#2626

Issue To Be Solved

Currently, there is no Atree function to swap two elements of an Array, so Cadence needs to do a remove and insert which can be inefficient.

Thanks @bluesign for bringing this up at onflow/cadence#2626! 👍

Suggested Solution

Add Array.Swap() to swap two elements in place.

[turbolent]

@fxamacker How much work would it be to add such a function?

[fxamacker]

@fxamacker How much work would it be to add such a function?

@turbolent About 2-3 uninterrupted person-days to add function and tests.

[fxamacker]

@turbolent Is this feature still needed?

I can re-estimate work required because of changes in atree v0.8.0 (e.g. atree inlining & deduplication, etc.).

[RZhang05]

Is this feature still needed?

Still relevant as an optimization for Array.reverse, but not high prio just nice to have.