pipeline error routing

macros

Reintroduce `r_print!` and `r_println!` macros as public exports, now
supporting both explicit buffer argument and implicit `#[buffer]` attr.
Add `#[buffer]` attribute macro that wraps unit-returning functions to
produce a `RenderResult` with an automatically injected buffer variable.
Relax `RenderResult::print()` and `println()` to accept `impl
AsRef<str>`.

Use the span of the `?` token when generating the macro invocation in
the `#[routeify]` transformation, so that rust-analyzer resolves hover
and go-to-definition on `?` to the `route!` macro's documentation