Use cats-effect in core

[johnynek]

Cats effect works on all scala platforms now. No reason not to use it in core and we can avoid some of the tagless and also the Par wrapper for parallelism.

[johnynek]

when this is done should also pass a config class to control the mode we are compiling.

For instance, we don't need to normalize during check, we are just checking that we can infer and that lints pass.

[johnynek]

this is a bit more complex than "just use cats-effect". In scala-js we can't block, so we can't run unsafeRunSync, so that means we have to fixup test code to handle the async result.