Hash-cons BDD descriptors

[gldubc]

Add hashes to every bdd nodes and bdd literal. Goal is to speed up comparisons, allow additional deep optimizations that test equality, and enable future caching of bdd operations.

Perfs:
this branch's perf:
The only real difference with main (included below) is that openapispex compiles much faster thanks to an added optimization.

| Codebase | LoC | Files | Modules | TC Time |
|---|---:|---:|---:|---:|
| Livebook | 65,218 | 264 | 266 | 0.180 s |
| HexPm | 33,360 | 282 | 301 | 0.221 s |
| Ecto | 32,804 | 56 | 147 | 0.034 s |
| PhoenixLiveView | 29,682 | 55 | 97 | 0.025 s |
| Credo | 29,199 | 262 | 327 | 0.065 s |
| Phoenix | 22,823 | 74 | 110 | 0.027 s |
| OpenApiSpex | 8,613 | 80 | 103 | 0.080 s |
| ExDoc | 7,297 | 30 | 36 | 0.013 s |
| Nerves | 6,024 | 51 | 52 | 0.011 s |
| Spitfire | 4,876 | 5 | 7 | 0.171 s |
| SQL | 3,782 | 18 | 19 | 0.029 s |
| AbsintheFederation | 2,158 | 18 | 23 | 0.008 s |

for comparison, main perf:

| Codebase | LoC | Files | Modules | TC Time |
|---|---:|---:|---:|---:|
| Livebook | 65,218 | 264 | 266 | 0.176 s |
| HexPm | 33,360 | 282 | 301 | 0.288 s |
| Ecto | 32,804 | 56 | 147 | 0.025 s |
| PhoenixLiveView | 29,682 | 55 | 97 | 0.029 s |
| Credo | 29,199 | 262 | 327 | 0.090 s |
| Phoenix | 22,823 | 74 | 110 | 0.030 s |
| OpenApiSpex | 8,613 | 80 | 103 | 0.244 s |
| ExDoc | 7,297 | 30 | 36 | 0.015 s |
| Nerves | 6,024 | 51 | 52 | 0.010 s |
| Spitfire | 4,876 | 5 | 7 | 0.131 s |
| SQL | 3,782 | 18 | 19 | 0.027 s |
| AbsintheFederation | 2,158 | 18 | 23 | 0.006 s |

The optimization mentioned above which helps openapispex, is l. 6135 optimization {_, _, bdd, u, bdd} -> bdd_union(bdd, u) for bdd intersection. It brings down TC times from 280ms to 60ms with no change for others.

The same l. 5828 opti (for bdd union) has a small or negligible impact overall, same for l. 5916 (for bdd difference).

Another note: since we have hashes now and it's cheap, I added simple equality checks for bdd union and intersection, e.g. bdd_union(bdd, bdd), do: bdd