Remove blanket Render impl for T: Display

[lambda-fairy]

While this is convenient, it's problematic because:

1. Weird edge cases. See impl Render for borrows ?  #266. While that particular case could be solved with an impl Render for &PreEscaped<T>, I'd rather go for a more general solution.
2. Performance. The std::fmt machinery is pretty heavyweight, and building Render on top of it locks us out of making it faster. In fact, this is the reason why markup.rs beats Maud in benchmarks – they use the itoa crate to render integers.
3. Semantics. There's no particular reason why the text and HTML representations should be tied together.

As part of this change, we should:

• Add manual impls for integer types.
• Investigate impact on users like miniserve (and maybe send them a migration PR)