Subst improvements

[mtzguido]

I want to measure this across more files, but here's the improvement on the repro from #3800.

Before:

Stats:
                   key     calls      tree     point       exn
             norm_term   1471862  43374 ms  43374 ms      0 ms
           tc_one_file         8  60415 ms   8859 ms      0 ms
             norm_comp         4   8179 ms   8179 ms      0 ms
    Parser.Dep.collect         1      4 ms      3 ms      0 ms
                 parse         2      1 ms      1 ms      0 ms
           simplify_vc      1290  33070 ms      0 ms      0 ms
         desugar_decls         1      0 ms      0 ms      0 ms
      deep_compress_se        15      0 ms      0 ms      0 ms
      hash_dependences         7      0 ms      0 ms      0 ms
ramon: group.total          60.918s
ramon: group.mempeak        10874MiB

After:

Stats:
                   key     calls      tree     point       exn
             norm_term   1471862  38437 ms  38437 ms      0 ms
           tc_one_file         8  54074 ms   9224 ms      0 ms
             norm_comp         4   6410 ms   6410 ms      0 ms
    Parser.Dep.collect         1      5 ms      4 ms      0 ms
                 parse         2      1 ms      1 ms      0 ms
           simplify_vc      1290  27164 ms      0 ms      0 ms
         desugar_decls         1      0 ms      0 ms      0 ms
      deep_compress_se        15      0 ms      0 ms      0 ms
      hash_dependences         7      0 ms      0 ms      0 ms
ramon: group.total          54.406s
ramon: group.mempeak        6880MiB

So reducing RAM usage by 1/3.

https://github.com/mtzguido/FStar/actions/runs/16949854112