replace pool indices with enum

[blairmunroakusa]

Per KS-INT-11

This is straight forward:

• [ ] ~introduce enum~
• [ ] ~replace in ilockmvp main~
• [ ] ~replace in tests~

Instead create guards for index range.

[blairmunroakusa]

Not possible to pass enum values to function, so the enum approach is not valid.

[blairmunroakusa]

Will instead include guards to check for out of bound indices.

[blairmunroakusa]

Complete via PR 232.