Unified errors

[udoprog]

This unifies the error hierarchy for compilation under rune::compile, which simplifies a bunch of code.

Also introduces a public WithSpan trait, that can be used by macro authors to attach a custom span to an error of their choosing.

With this, a complete compiler error translated from something like an Option::None can be written as:

use rune::compile::WithSpan;

let value = option.ok_or("missing thing").with_span(span)?;