specs/bytestring: strengthen unsafeTail specs
specs/bytestring: strengthen unsafeTail specs

This now specifies that the output ByteStrin is exactly one byte smaller
than the input, vs merely having the predicate that the input string has
to have a non-zero length.

Signed-off-by: Austin Seipp <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell