Fix #86 - make randomWord produce uniform randoms

When :check was executed on a function accepting words as arguments
cryptol would generate mostly small words.  This strategy failed to find
counter-examples in formulas such as:

    (\a b -> (((a : [16]) + b) % 17) == ( (a % 17 + b %17) % 17 ) )

This failure is particularly egregious considering 50% of the inputs
result in False.  We now generate a better distribution of words and are
discussing use of a smart-check like strategy or an evaluation of the
logic to tailor generation in a future release (see issue #87).