ctfe: limit hashing of big const allocations when interning
Const allocations are only hashed for interning. However, they can be large, making the hashing expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially big buffers like the actual bytes of allocation and the associated 1/8th sized `InitMask`.
We can partially hash these fields when they're large, hashing the length, and head and tail of these buffers, to
limit possible collisions while avoiding most of the hashing work.
r? `@ghost`
43c47db0b0