Remove parameter perm_inv. perm_inv overcomplex the process for gener…

[zpcore]

We are generating the arena array to jump between different data segment/stride. Array perm tells which segment (stride) to choose and array mixer tells which element in each segment (stride) we are going to use. Below is simplified the example with nr_elts=6:
perm: 2 3 1 0 4 5.

The final goal is to build jumping in arena array: Segment 2-> segment 3 -> segment 1 -> segment 0 -> segment 4 -> segment 5.
With perm_inv, we will build the jumping data in arena with the following sequence:
segment 0 -> segment 4,
segment 1 -> segment 0,
segment 2 -> segment 3,
segment 3 -> segment 1,
segment 4 -> segment 5,
segment 5 -> segment 2.

Actually it is unnecessary to follow the construction order from 0 to 5. We can just loop through perm and build the jumping data directly:
segment 2 -> segment 3,
segment 3 -> segment 1,
segment 1 -> segment 0,
segment 0 -> segment 4,
segment 4 -> segment 5,
segment 5 -> segment 2.

Both solution will result in the same arena data content but with different sequence to fill the arena array.

Thus we can remove the perm_inv to simplify the code logic for generating the arena array, which also reduces the memory usage.

Results have been verified with puj2@.

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[zpcore]

Ping to see if anyone can help review the PR.