impl_ macros assume too much about scope

[gavofyork]

From a compile error when impl_outer_log is used without having codec::Input in scope:

error[E0405]: cannot find trait `Input` in this scope
   --> runtime/src/lib.rs:249:1
    |
249 | / impl_outer_log! {
250 | |     pub enum Log(InternalLog: DigestItem<SessionKey>) for Runtime {
251 | |         consensus(AuthoritiesChange)
252 | |     }
253 | | }
    | |_^ not found in this scope
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

It should work regardless of the scope of the macro invoker.