Add imap and iemap to Codec

[ybasket]

Adding an instance of cats.Invariant gives access to imap, further iemap is added on Codec itself for cases where "decoding" can fail. Handy for AnyVals and similar wrapper types.

[travisbrown]

Sorry for the long delay in responding. I've been reluctant to add something like this, because it felt like a dangerous step down a slippery slope toward thinking of Codec as a type class, and not just a thing that makes defining Encoder and Decoder instances a little more convenient.

I've been thinking about it a little more, though, and I'm not sure it's that big a deal. I'm inclined to go ahead and merge this.

[ybasket]

No problem. I know you're busy working on awesome stuff like dhallj, so I'm fine waiting :)

I definitely see your point and wouldn't be disappointed if the PR is declined for that - these methods can be useful in some cases, but it's also limited as you often don't have a "base" Codec at hand, so you don't get to use imap.

There's some prior art in doobie, Meta is similar to Codec (with differences, for example it has primitive instances itself) and has imap: https://github.com/tpolecat/doobie/blob/master/modules/core/src/main/scala/doobie/util/meta/meta.scala