Add Stable APIs to emit diagnostic messages

[celinval]

Provide a mechanism to emit diagnostic messages using StableMIR's Span. Maybe provide methods to emit error, warning, and note.
One possibility would be to provide an API similar to what is being proposed for proc_macro diagnostics:

// crate stable_mir

pub fn span_error(span: impl Spans, message: impl Message) -> proc_macro::Diagnostic;
pub fn span_warn(span: impl Spans, message: impl Message) -> proc_macro::Diagnostic;
pub fn span_note(span: impl Spans, message: impl Message) -> proc_macro::Diagnostic;

/// This could become an alias to proc_macro::Message once it gets added.
pub trait Message;

pub trait Spans {
    /// Converts `self` into a `Vec<Span>`.
    fn into_spans(self) -> Vec<Span>;
}

impl Message for String;
impl Message for &str;
impl Spans for Span;
impl<I: IntoIterator<Item = Span>> Spans for I;
impl<T> proc_macro::MultiSpan for T where T: Spans;

We should probably also add a function abort_if_errors() to allow tool developers to end some validation stage if any error was detected.

[oli-obk]

We could attempt do this entirely on crates.io with just the span information. There are crates that render diagnostics in the rustc style.

I'll build a small demo.

[oli-obk]

Here's a very hacky example of how to use annotate_snippets to render a diagnostic: https://github.com/oli-obk/smir_diagnostics

the demo produces the following output:

[celinval]

That's awesome! I didn't know the annotate-snippets crate before.

[celinval]

Maybe we can add methods to retrieve the byte range of a span and the file content, so users can easily integrate with a diagnostic crate such as annotate-snippets or codespan-reporting.