@inline Predef.identity[A].

This is a bytecode micro-optimization, but I don't see any issues with including it.

It helps in the case of something like:

def foo(a: (A)=>A): ...

foo(identity[A] _)

Commit text:
===
Since identity[A] is often used as a function directly supplied to
methods requiring (A)=>A, there's no need to have the generated
Function1 object contain an explicit call to identity[A].

This implementation isn't going to change, so let the synthetic
Function1 elide the extra call, generating just "aload_1; areturn".