Signed version of prelude function `toInteger`

[brianhuffman]

The Cryptol prelude has a function toInteger that can convert a bitvector of type [n] to a non-negative integer in the range 0 to 2^^n-1. We need a signed variant of this function that converts a signed bitvector to an integer in the range -2^^(n-1) to 2^^(n-1)-1. Such a function exists in the saw prelude (called sbvToInt), but not in Cryptol.