Making StreamingHandler support conditional Send

[kornelski]

I've tried making StreamingHandler support Send conditionally, but it's not feasible without major rewrites. In the current architecture, the conditional Send bound becomes incredibly viral and makes a total mess of the whole internal API. The problem is that the handler is in Mutations, which makes it Mutations<H>, and the mutations are in the StartTag/EndTag structs (now StartTag<H>), which are in Token, and the Token<H> is used all over the place. The TagScanner and Lexer are tied to Token<H>, so they become Lexer<H> too. It spreads everywhere.

I think this is fixable — Mutations are not used until late in the pipeline, so the parsing stage could be changed to use another type that is something in between TokenOutline and a Token, and the types like StartTag would be (AlmostToken, Mutations<H>). However, such refactoring will touch a lot of the parser, dispatcher, token capturer. I'd prefer to release as-is to keep changes between releases small, and add non-Send support later.