Include span label in rust lints (#601)

* Include span label in rust lints

This turns relatively unhelpful error messages like

    mismatched types

into more expressive messages along the lines of

    mismatched types: expected bool, found integral variable

Fixes #597.

* Exclude rust lint span label if empty

* Use single-quoted strings in vimscript

* Add test for detailed rust errors

* Prune Cargo JSON

* Use matching error file name

* Byte offsets not char offsets

autoload/ale/handlers/rust.vim

@@ -55,7 +55,7 @@ function! ale#handlers#rust#HandleRustErrorsForFile(buffer, full_filename, lines
                 \   'end_lnum': l:span.line_end,
                 \   'col': l:span.byte_start,
                 \   'end_col': l:span.byte_end,
-                \   'text': l:error.message,
+                \   'text': empty(l:span.label) ? l:error.message : printf('%s: %s', l:error.message, l:span.label),
                 \   'type': toupper(l:error.level[0]),
                 \})
             else