repr: improve layout table printing

This commit improves the layout table printing by removing the empty gap
at the end of a row for non-single element rows. This means that previously
the layout table would print for a struct like this:

```rs
\#layout_of Boon := enum(
    Foo(x: i32, y: i32),
    Faz(z: f64),
);
```

Before:

```
info: Layout of `Boon` (size=16b align=8b):
┌──────────────┬─────────────────────────┐
│ 4b tag (u32) │                         │
├──────────────┼────────────┬────────────┤
│     Foo      │   x: i32   │   y: i32   │
│              │            │            │
│      0       │   size: 4b │   size: 4b │
│              │ offset: 4b │ offset: 8b │
│              │  align: 4b │  align: 4b │
├──────────────┼────────┬───┴────────┬───┤
│     Faz      │ 4b pad │   z: f64   │   │
│              │        │            │   │
│      1       │   ##   │   size: 8b │   │
│              │   ##   │ offset: 8b │   │
│              │   ##   │  align: 8b │   │
└──────────────┴────────┴────────────┴───┘
```

After the change, we now print the following:

```
info: Layout of `Boon` (size=16b align=8b):
┌──────────────┬─────────────────────────┐
│ 4b tag (u32) │                         │
├──────────────┼────────────┬────────────┤
│     Foo      │   x: i32   │   y: i32   │
│              │            │            │
│      0       │   size: 4b │   size: 4b │
│              │ offset: 4b │ offset: 8b │
│              │  align: 4b │  align: 4b │
├──────────────┼────────┬───┴────────────┤
│     Faz      │ 4b pad │     z: f64     │
│              │        │                │
│      1       │   ##   │     size: 8b   │
│              │   ##   │   offset: 8b   │
│              │   ##   │    align: 8b   │
└──────────────┴────────┴────────────────┘
```

compiler/hash-repr/src/write.rs

@@ -338,8 +338,10 @@ impl BoxRow {
             return;
         }
 
-        // Insert an empty box to fill in the missing gap.
-        if width - current_width > 2 {
+        // For singletons, insert an empty box to fill in the missing gap.
+        let singleton = self.contents.len() == 1;
+
+        if singleton && width - current_width > 2 {
             let empty_width = width - current_width - 1;
             self.widths.push(empty_width);
             self.contents.push(BoxContent::new_empty(empty_width));

tests/cases/typecheck/enums/discriminants/valid.hash

@@ -61,6 +61,10 @@ Variants := enum(
 #layout_of
 Empty := enum();
 
-
+#layout_of
+Boon := enum(
+    Foo(x: i32, y: i32),
+    Faz(z: f64),
+);
 
 main := () => {}

tests/cases/typecheck/enums/discriminants/valid.stdout

@@ -102,3 +102,20 @@ info: Layout of `!` (size=0b align=1b):
 │  align: 1b │
 └────────────┘
 
+info: Layout of `Boon` (size=16b align=8b):
+┌──────────────┬─────────────────────────┐
+│ 4b tag (u32) │                         │
+├──────────────┼────────────┬────────────┤
+│     Foo      │   x: i32   │   y: i32   │
+│              │            │            │
+│      0       │   size: 4b │   size: 4b │
+│              │ offset: 4b │ offset: 8b │
+│              │  align: 4b │  align: 4b │
+├──────────────┼────────┬───┴────────────┤
+│     Faz      │ 4b pad │     z: f64     │
+│              │        │                │
+│      1       │   ##   │     size: 8b   │
+│              │   ##   │   offset: 8b   │
+│              │   ##   │    align: 8b   │
+└──────────────┴────────┴────────────────┘
+