Fix `Text.FastSet` - Githubissues

Otherwise consider the case of,

let fs = fromList "abcx"
in '\239' `member` fs

In this case,

l == 4, therefore
((5 * l)div4) == 5, therefore
mask' == 7, therefore
we ended up with a hashtable array of length 14
however, during the membership check we start with i = 239 .&. 7 = 7
meaning the first entrance into go starts with j = 14; the first lookupAt tries to examine elements 14 and 15 of the hashtable array, which are out-of-bounds.

The solution taken here is to ensure there is always at least one entry of padding at the end of the hashtable array. This now passes the quickcheck tests added previously.

FastSet carries a quite nice performance improvement in microbenchmarks,

benchmarking sets/Fast
time                 8.046 ns   (8.011 ns .. 8.115 ns)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 8.076 ns   (8.042 ns .. 8.130 ns)
std dev              135.4 ps   (98.67 ps .. 193.0 ps)
variance introduced by outliers: 24% (moderately inflated)

benchmarking sets/Hash
time                 15.18 ns   (15.13 ns .. 15.24 ns)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 15.15 ns   (15.13 ns .. 15.19 ns)
std dev              94.77 ps   (62.22 ps .. 155.6 ps)

benchmarking sets/Int
time                 14.11 ns   (14.03 ns .. 14.22 ns)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 14.22 ns   (14.16 ns .. 14.28 ns)
std dev              210.7 ps   (184.2 ps .. 250.6 ps)
variance introduced by outliers: 19% (moderately inflated)

benchmarking sets/TextFast
time                 8.532 ns   (8.482 ns .. 8.572 ns)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 8.485 ns   (8.461 ns .. 8.520 ns)
std dev              101.8 ps   (81.45 ps .. 161.0 ps)
variance introduced by outliers: 14% (moderately inflated)

Fixes #103.

haskell / attoparsec

Fix `Text.FastSet` #118