consider using proc-macro-error to simplify the emission of error messages

[japaric]

The analysis pass collects error messages manually like this:

rtic-syntax/src/analyze.rs

Lines 109 to 115 in b5389b4

	error.push(syn::Error::new(
	r.span(),
	format!(
	"Lock free shared resource {:?} is used by tasks at different priorities",
	r.to_string(),
	),
	));

The proc-macro-error crate provides similar functionality: it lets you emit rustc-like error, notes and warnings messages with appropriate spans. It has an emit_error! API to collect error messages and a separate abort! API to terminate the proc-macro and emit all collected errors. I think using proc-macro-error API would simplify the error handling in the analysis pass.