serialize: Change some FnOnce bounds to FnMut

Relax some of the bounds on the decoder methods back to FnMut to help accomodate
some more flavorful variants of decoders which may need to run the closure more
than once when it, for example, attempts to find the first successful enum to
decode.

This a breaking change due to the bounds for the trait switching, and clients
will need to update from `FnOnce` to `FnMut` as well as likely making the local
function binding mutable in order to call the function.

[breaking-change]
Merge pull request #19887 from alexcrichton/serialize-fn-mut

serialize: Change some FnOnce bounds to FnMut

Reviewed-by: brson