Add Show instance for Core.

As shortly discussed with @joshcough on #ermine, here's the `Show` instance for `Core`.
I kept `toString` for `Var`, `Data`, and `App` for consistency with the other `Core` data constructors that are implemented as case classes.

Notice that the `PrimFun`s `body: Any` field is just printed with `toString`. Or maybe it should just print the string `<body>`?

Cheers