Allow caching Clifford+T synthesis results

Circuits can frequently re-use the same rotation angle, e.g. QFTs or Suzuki-Trotter expansions of static Hamiltonians. We currently re-compute the Clifford+T synthesis of same angles, even though the same computation was already performed. We should allow caching the results to drastically reduce runtime in these cases. (This require care since we'd be caching based on floating point numbers...)