Add Stable APIs to emit diagnostic messages

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.

Start by reviewing the proposed proc_macro diagnostics API and the existing StableMIR Span concept. Define the stable APIs for error, warning, and note messages, including the Message and Spans traits, and decide how abort_if_errors should behave. Done means the diagnostic mechanism and validation-stage behavior are specified and implemented; no files or tests are named in the issue.