Make fn argument name errors better (#1238)

### What
When displaying argument name errors highlight the specific argument
name instead of the fn as a whole as the point of the error.

### Why
So that it is easier to see immediately where the error occurs.

There are no test changes. I don't love that there isn't test coverage
for compile errors. We used to have them, but removed all compile error
testing at some point because in general the impact of breaking compile
errors is low, and the tests are very slow because the tests have to run
through a full compile and capture compiler output.

### Example

#### Before

```
error: argument name too long, max length 30 characters
 --> tests/empty/src/lib.rs:9:12
  |
9 |     pub fn empty(a234567890123456789001234567890: u64) {}
  |            ^^^^^
```

### After

```
error: argument name too long, max length 30 characters
 --> tests/empty/src/lib.rs:9:18
  |
9 |     pub fn empty(a234567890123456789001234567890: u64) {}
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

soroban-sdk-macros/src/derive_spec_fn.rs

@@ -66,7 +66,7 @@ pub fn derive_fn_spec(
                         let name = name.try_into().unwrap_or_else(|_| {
                             const MAX: u32 = 30;
                             errors.push(Error::new(
-                                ident.span(),
+                                a.span(),
                                 format!("argument name too long, max length {} characters", MAX),
                             ));
                             StringM::<MAX>::default()