Fix tests on 32 bit platforms

The library was failing tests on my 32 bits machine.

Two tests were failing:

1.  prop_showRead :: Day -> Bool. This is just because Day in Int64 internally and YearMonthDay has an Int as year. I limited the range of Days we test to Int.

2. prop_parseTime. This is a real bug; you were parsing '%s' as an Int, which introduced a year 2038 bug to your code.

We did the debugging together with @errge.