Fast and correct aeson instances for UTCTime

[ondrap]

This patch does several things:

• adds benchmark for Thyme aeson UTCTime encode and parse vs. Time instances
• adds faster encoding/parsing instances for UTCTime: they used to be ~ twice slower than time, now it is ~2 times faster. I basically copied the encoding instance from aeson and I have written the parsing myself.
• adds quickcheck tests for these instances; the instances are correct (i.e. decode (encode time)) == Just time for time between 1AC-9999AC).
• fixes problems with compilation with older versions of GHC, now it passes the Travis checks completely for all GHC versions
• I didn't touch the ZonedTime instance as it seemed to me it parsed more formats than I would expect (the RFC3339) - if you feel I should change that, send me a note and I could do that.
• I have added dependency on the scanner package; the parser is about twice as fast as an identical attoparsec parser, I think it makes sense

Benchmark results:

benchmarking time encode/time/encode
time                 1.097 μs   (1.058 μs .. 1.146 μs)
                     0.990 R²   (0.981 R² .. 0.998 R²)
mean                 1.072 μs   (1.049 μs .. 1.115 μs)
std dev              96.30 ns   (57.69 ns .. 168.7 ns)
variance introduced by outliers: 86% (severely inflated)

benchmarking time encode/thyme/encode
time                 547.6 ns   (537.6 ns .. 566.3 ns)
                     0.992 R²   (0.981 R² .. 0.999 R²)
mean                 558.1 ns   (545.7 ns .. 583.9 ns)
std dev              55.11 ns   (35.69 ns .. 89.25 ns)
variance introduced by outliers: 89% (severely inflated)

benchmarking time decode/time/decode
time                 2.467 μs   (2.438 μs .. 2.493 μs)
                     0.999 R²   (0.998 R² .. 0.999 R²)
mean                 2.475 μs   (2.448 μs .. 2.506 μs)
std dev              100.6 ns   (82.19 ns .. 128.5 ns)
variance introduced by outliers: 54% (severely inflated)

benchmarking time decode/thyme/decode
time                 856.0 ns   (850.2 ns .. 862.2 ns)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 859.9 ns   (852.5 ns .. 868.4 ns)
std dev              25.96 ns   (20.86 ns .. 33.31 ns)
variance introduced by outliers: 42% (moderately inflated)