They used to be instance fields, which made a compiled expression unsafe to call
from more than one thread at a time: two calls interleaved their pushes and pops
and the second to finish found the stack in a state it did not put it in. Worse,
the damage was permanent -- the count check throws before anything is popped, so
one racing call left the leftovers behind and every later call failed too, on one
thread or many. See https://github.com/asc-community/AngouriMath/issues/637.
Per thread rather than per call so that calling a compiled expression still
allocates nothing, which is the point of compiling it. Neither buffer carries
anything from one call to the next: the stack is emptied on the way in, and the
cache is only ever read at a slot the same call has already written.