Move finish out of the Encoder trait.

This simplifies things, but requires making `CacheEncoder` non-generic.

(This was previously merged as commit 4 in #94732 and then was reverted
in #97905 because it caused a perf regression.)

src/librustdoc/scrape_examples.rs

@@ -18,7 +18,7 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_serialize::{
     opaque::{FileEncoder, MemDecoder},
-    Decodable, Encodable, Encoder,
+    Decodable, Encodable,
 };
 use rustc_session::getopts;
 use rustc_span::{