[proc-macro] Add possibility to generate a `Span` from a path representation

Currently, we cannot handle Span when using the proc-macro crate, which is problematic in many cases, like askama when the generated code triggers an error and we can't show the users where in their templates the error originates from.

So an idea I had was to provide a method looking like this:

struct FileLocation {
    line: u32,
    column: u32,
}

fn span_from_representation(file: &Path, beginning: FileLocation, end: Option<FileLocation>) -> Option<Span>;

It would allow to get the span we need without having to manipulate the Span content (which is not possible outside of the compiler anyway).

Start by reviewing proc-macro Span handling and the proposed span_from_representation signature in the issue. Done means providing a way to derive an Option from a path and beginning/end file locations so generated errors can point to their source templates; no files or tests are identified.