Optimize pairing verification

[jannikluhn]

See

shutter/shlib/shcrypto/keys.go

Lines 203 to 205 in 31707b7

	// TODO: This could be optimized by inverting one of the arguments in one of the pairings,
	// multiplying the results and comparing the result to 1. This would improve performance as it
	// avoids one of the final exponentiations.