Description:

u8 is enough for this field. Actually there are two more optimization we probably can do:

1. unresolved_ctxt is rarely changed so we can avoid copying it when copying ExprCtx.
2. On the top of point 1, the remaining fields can be compacted into a u8

But I don't want to do them, since they will make code messier and less extendable.
This pr makes ExprCtx from 12 bytes to 8 bytes. 8 bits = 64 bits is good enough for now.