Fix pretty printing long units with long prefixes

Before this, we had:

```
>>> @metric_prefixes
unit points

  @metric_prefixes
  unit points

>>> megapoints

  megapoints

    = 1 Mpoints
```

Now we have:

```
>>> @metric_prefixes
unit points

  @metric_prefixes
  unit points

>>> megapoints

  megapoints

    = 1 megapoints
```

I've also added a roundtrip test.

numbat/src/typed_ast.rs

@@ -616,6 +616,9 @@ mod tests {
                  @aliases(in: short)
                  unit inch = 0.0254 m
 
+                 @metric_prefixes
+                 unit points
+
                  let a = 1
                  let b = 1
                  let c = 1
@@ -731,6 +734,7 @@ mod tests {
         roundtrip_check("2^3!");
         roundtrip_check("-3!");
         roundtrip_check("(-3)!");
+        roundtrip_check("megapoints");
     }
 
     #[test]

numbat/src/unit.rs

@@ -193,13 +193,18 @@ impl Power for UnitFactor {
     }
 }
 
-// TODO: Use the AcceptsPrefix here to format the string correctly
 impl Display for UnitFactor {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let prefix = if self.unit_id.canonical_name.accepts_prefix.short {
+            self.prefix.as_string_short()
+        } else {
+            self.prefix.as_string_long()
+        };
+
         write!(
             f,
             "{}{}{}",
-            self.prefix.as_string_short(),
+            prefix,
             self.unit_id.canonical_name.name,
             pretty_exponent(&self.exponent)
         )