Intersective pattern matching

[texttheater]

E.g.:

for Obj<a: Num> def f Num as is Obj<b: Num> then @a + @b else @a ok ok

Currently gives a type error because Obj<a: Num> does not subsume Obj<b: Num>; but matching should just result in the intersection of both types. Similarly Nearr/Nearr, Arr/Arr, Nearr/Arr intersections are possible.

Depends on #4 (specifically, the Partition method).