Perf optimizations and inferred intersections

[gldubc]

• 1. Memoize phi().
• 2. Fast path: if there are no none() in the arguments of an intersection, we can compute subtyping quicker using fun_apply
• 3. Remove duplicates in tuple_difference (reuses tuple_union logic).

2. and 3. are sufficient to make the fun_from_inferred_clauses work.
3. will be necessary for large function intersections with empty arguments.

Also adds inferred intersection types, in fun_from_inferred_clauses.

[josevalim]

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