Implement JPA opcode

[jamesyoungman]

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[jamesyoungman]

This is a reasonably good starter issue because we already implemented SKM (in op_jump.rs) and this jump instruction will have a similar instruction, but a simpler condition (the comparison part is already implemented in the base crate, too).

JPA is described on page 3-32 of the Users Handbook: http://www.bitsavers.org/pdf/mit/tx-2/TX-2_UserHandbook_ch3.pdf