Use a single fold for difference

[JordanMartinez]

Description of the change

Uses a single fold for difference rather than folding over m2 once to produce a list and then folding over that list a second time to do the deletion.

Benchmark below:

Map
===
difference
---------------
Map2a0bff.difference: small map (100)
mean   = 17.89 ms
stddev = 1.37 ms
min    = 16.17 ms
max    = 40.21 ms
M.difference: small map (100)
mean   = 2.43 ms
stddev = 921.62 μs
min    = 1.84 ms
max    = 18.75 ms
Map2a0bff.difference: midsize map (10000)
mean   = 54.24 ms
stddev = 939.99 μs
min    = 53.27 ms
max    = 57.81 ms
M.difference: midsize map (10000)
mean   = 32.48 ms
stddev = 6.42 ms
min    = 24.70 ms
max    = 50.55 ms
Map2a0bff.difference: big map (1000000)
mean   = 65.23 ms
stddev = 3.98 ms
min    = 60.30 ms
max    = 72.10 ms
M.difference: big map (1000000)
mean   = 48.51 ms
stddev = 4.39 ms
min    = 45.34 ms
max    = 60.00 ms

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)