der: add ApplicationExplicit, ContextSpecificExplicit, PrivateExplicit

[dishmaker]

Partially fixes

• der: drop EncodeValue, DecodeValue trait requirements for EXPLICIT #1745

Derive macro support in future PR.

[dishmaker]

This partially fixes #1808, because we can rewrite derive(Choice) as:

match tag {
    String::TAG => {}
    // ContextSpecificExplicit knows it's constructed, without trickery
    ContextSpecificExplicit::<2, BitString>::TAG => {}
    // .. more choices
}

[tarcieri]

I'm getting a little worried about the internal_macros module. It's a lot of macros. Perhaps there are some non-macro-based abstractions to explore there.

[dishmaker]

So my plan is to make this PR pure generic *Explicit.

CustomTagExplicit<TAG, T, CLASS>

Meanwhile, macro code will stay the same and will be eventually deprecated (because of trait flaws).