v0.5.7

This version contains two fixes (see #88) for issues discovered in rzk-lang/sHoTT#30:

1. We now only generate well-typed LEM instances in the tope solver, speeding up significantly.
2. We fix $\eta$-rule for product cubes, to not get stopped by reflexive equality topes like $\langle \langle \pi_1 (t_{12}), \pi_2 (t_{12}) \rangle, t_3 \rangle \equiv \langle t_{12}, t_3 \rangle$.