@inline Predef.identity[A].

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".
Merge pull request #3737 from tvierling/identity-inline

@inline Predef.identity[A].