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`.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/value.zig b/src/value.zig
index 1a2e85bb1e..fdf061c680 100644
--- a/src/value.zig
+++ b/src/value.zig
@@ -426,7 +426,7 @@ pub const Value = struct {
                     // Assume it is already an integer and return it directly.
                     .simple_type, .int_type => val,
                     .enum_type => |enum_type| if (enum_type.values.len != 0)
-                        enum_type.values[field_index].toValue()
+                        enum_type.values.get(ip)[field_index].toValue()
                     else // Field index and integer values are the same.
                         mod.intValue(enum_type.tag_ty.toType(), field_index),
                     else => unreachable,