Aeson 2 compatibility

[michaelpj]

We get compatibility with both <2 and >=2 by using only functions that
appear in both, which don't make assumptions about the structure of
objects.

Fixes #356

[jneira]

Looks great and elegant with no CPP conditions (with only some scary lens-like operators in exchange :-P)

@fendor do you think we could remove cpp conditions from hie-bios as well?

[michaelpj]

Oh and just to confirm: I built this with both versions 1.5.6 and 2.0.1, so it does definitely build with both.

[fendor]

Looks great and elegant with no CPP conditions (with only some scary lens-like operators in exchange :-P)

@fendor do you think we could remove cpp conditions from hie-bios as well?

Sounds like a good idea, will try it