InternPool: safer enum API

The key changes in this commit are:

```diff
-        names: []const NullTerminatedString,
+        names: NullTerminatedString.Slice,
-        values: []const Index,
+        values: Index.Slice,
```

Which eliminates the slices from `InternPool.Key.EnumType` and replaces
them with structs that contain `start` and `len` indexes. This makes the
lifetime of `EnumType` change from expiring with updates to InternPool,
to expiring when the InternPool is garbage-collected, which is currently
never.

This is gearing up for a larger change I started working on locally
which moves union types into InternPool.

As a bonus, I fixed some unnecessary instances of `@as`.

src/Module.zig

@@ -6655,7 +6655,7 @@ pub fn enumValueFieldIndex(mod: *Module, ty: Type, field_index: u32) Allocator.E
 
     return (try ip.get(gpa, .{ .enum_tag = .{
         .ty = ty.toIntern(),
-        .int = enum_type.values[field_index],
+        .int = enum_type.values.get(ip)[field_index],
     } })).toValue();
 }