Auto merge of #98160 - nnethercote:mv-finish-out-of-Encoder, r=bjorn3

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.)

r? `@ghost`

Author: bors