Auto merge of #30064 - fhartwig:macro-suggestions, r=sanxiyn

Fixes #13677
This does the same sort of suggestion for misspelt macros that we already do for misspelt identifiers.
Example. Compiling this program:

```rust
macro_rules! foo {
    ($e:expr) => ( $e )
}

fn main() {
    fob!("hello!");
}
```

gives the following error message:

```
/Users/mcp/temp/test.rs:7:5: 7:8 error: macro undefined: 'fob!'
/Users/mcp/temp/test.rs:7     fob!("hello!");
                              ^~~
/Users/mcp/temp/test.rs:7:5: 7:8 help: did you mean `foo`?
/Users/mcp/temp/test.rs:7     fob!("hello!");
```

I had to move the levenshtein distance function into libsyntax for this. Maybe this should live somewhere else (some utility crate?), but I couldn't find a crate to put it in that is imported by libsyntax and the other rustc crates.

src/libsyntax/lib.rs

@@ -65,6 +65,7 @@ macro_rules! panictry {
 
 pub mod util {
     pub mod interner;
+    pub mod lev_distance;
     pub mod node_count;
     pub mod parser;
     #[cfg(test)]