Remove heap allocations from AST set with open addressing.
Remove heap allocations from AST set with open addressing.

This removes about 15% of the mallocs and speeds up the set operations
roughly the same.

Signed-off-by: Teemu Koponen <514c492c49408016e397a2f741baa6dd9cf7f3be@styra.com>

open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa