PersistField Int should parse PersistRational

[parsonsmatt]

The instance PersistField Int does not work with the Postgres numeric type, which shows up repeatedly for the esqueleto library.

Link to code.

We should really probably just make PersistRational work.

bitemyapp/esqueleto#82
bitemyapp/esqueleto#54
bitemyapp/esqueleto#175

[parsonsmatt]

Thoughts:

I don't like floor. That feels like... throwing away information that may be pertinent.

If the Rational is like x % 1 then I think it's totally fine to just return x directly.

But if it's x % y where the value isn't integral, then that feels like a Left to me.