Limit the amount of nesting for `thresh` fragments to 4

compute_thresh_ops and compute_thresh_stacksize each need two arrays of
MAX_N_IN_THRESH + 2 counters. Since they were inlined by the compiler,
they bloat the size of the stack frame of
compute_miniscript_policy_ext_info, which vastly reduces the stack usage
on usual policies.

Yet, stack usage would remain very large on policies that recursively
nest 'thresh' fragments.
Therefore, we add a limit of 4 nested thresh expressions, by keeping
track in the parsing context.