Rename toJson to asJson about circe

[travisbrown]

Yeah, that makes sense. I was thinking to made the fact that it's a conversion a little clearer, but I could go either way.

We might need to make some changes in the benchmarks as well, since I use toJson side by side with Argonaut's asJson—I don't know off the top of my head whether they'll work together if they have the same name.

[vkostyukov]

We can also go other way: use to[Foo] on jfc's Json instead of as[Foo]. But it will make it a bit harder for people to migrate from Argonaut to jfc.

[propensive]

My general rule is that a method called to takes a type constructor, whereas a method called as takes a proper type.

[travisbrown]

I think that to me to means conversion, and specifically conversion that can fail, while as is more like safe casting or interpretation—but that's exactly the opposite of my current API, so I'm clearly not thinking too consistently about this stuff.

I say let's go with @propensive's rule and @vkostyukov's suggestion and use as[A] and asJson.