Modify `define_encode_set` to support private definitions

[brson]

Right now all encode sets must be explicitly pub. Generally such macros should have a grammar that accepts non-pub definitions.

It doesn't look like the existing grammar supports this extension though:

define_encode_set! {
    /// This encode set is used in the URL parser for query strings.
    pub QUERY_ENCODE_SET = [SIMPLE_ENCODE_SET] | {' ', '"', '#', '<', '>'}
}

There's no other keyword there besides pub. Seems like this definition would be better if you had to type like pub struct QUERY_ENCODE_SET.

So fixing this might require adding two rules and basically deprecating the existing rule? Not sure.

[SimonSapin]

As discussed in #298 (comment) this is a very niche feature, so extending it seems low value. In particular it doesn’t seem worth adding new APIs and deprecating old ones, to me.

[brson]

@SimonSapin ok, sgtm.