Structural termination checker doesn't seem to always trigger on where-clauses

[Alex Gryzlov]

There's an occurrence of a helper definition that currently has to be annotated with a measure that is essentially emulating structural termination:

https://github.com/ucsd-progsys/liquidhaskell/blob/develop/tests/benchmarks/bytestring-0.9.2.1/Data/ByteString/Lazy.hs#L441