[mesh] [opt] Support nested mesh-for

[g1n0st]

Related issue = #3608

Before this PR, do the neighbor query loop in mesh-for must resort to nested range-for, like:

for u in model.verts:
  for i in u.verts.size:
    u.x += u.verts[i].y

This PR implemented the nested mesh-for loop, the code above could be rewritten as:

for u in model.verts:
  for v in u.verts:
    u.x += v.y

Note that the nested mesh-for loop would not be un-looped.

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[BillXu2000]

Looks good to me!