Reified `&Write<T>` barriers and `field!` projections.

[moulins]

This adds new APIs for safe field-level mutation:

• Add barrier module, with Write, field!, unlock!;
• Move Unlock into the barrier module;
• Rename Gc::write_barrier to Gc::write and make it return a &'gc Write<T>.

See the field_locks test for a simple example.