Experiment with subspan support

[Nemo157]

https://doc.rust-lang.org/nightly/proc_macro/struct.Literal.html#method.subspan

Seems to work well with a hacked together usage:

error[E0425]: cannot find value `a` in this scope
  --> tests/tests.rs:15:46
   |
15 |         assert_eq!(stylish::plain::format!("{a}"), "1");
   |                                              ^ not found in this scope

Biggest blocker seems to be detecting the quotes, for simple " quoted strings this works:

diff --git macros/src/lib.rs macros/src/lib.rs
index a328ed3..9fde367 100644
--- macros/src/lib.rs
+++ macros/src/lib.rs
@@ -107,9 +107,9 @@ fn format_args_impl(
     let export: syn::Path = syn::parse_quote!(#krate::𓀄);

     let span = format.span();
-    let format_string = &format;
-    let format = format.value();
-    let (leftover, format) = Format::parse(&format).unwrap();
+    let format_lit = &format;
+    let format_string = format.value();
+    let (leftover, format) = Format::parse(&format_string).unwrap();
     assert!(leftover.is_empty());
     let num_positional_args = positional_args.len();
     let positional_args = positional_args.into_iter();
@@ -161,12 +161,14 @@ fn format_args_impl(
                             quote!(__stylish_named_args.#index)
                         } else {
                             let i = implicit_named_args_values.len();
+                            let start = (name.as_ptr() as usize) - (format_string.as_str().as_ptr() as usize) + 1;
+                            let end = start + name.len();
                             implicit_named_args_values.push(ExprPath {
                                 attrs: Vec::new(),
                                 qself: None,
                                 path: Ident::new(
                                     name,
-                                    Span::call_site().resolved_at(format_string.span()),
+                                    format_lit.token().subspan(start..end).unwrap_or_else(|| format_lit.span()),
                                 )
                                 .into(),
                             });

But it gets misaligned for r#"" strings:

error[E0425]: cannot find value `a` in this scope
  --> tests/tests.rs:16:46
   |
16 |         assert_eq!(stylish::plain::format!(r#"{a}"#), "1");
   |                                              ^ not found in this scope

[Nemo157]

Another issue, escapes get interpreted making the offset off:

error[E0425]: cannot find value `a` in this scope
  --> tests/tests.rs:17:47
   |
17 |         assert_eq!(stylish::plain::format!("\u{0000}{a}"), "1");
   |                                               ^ not found in this scope

[Nemo157]

Probably blocked on dtolnay/syn#1219