Make isinstance more powerful

[nielstron]

This implements a number of improvements for isinstance handling

• isinstance is also a type cast for if expressions, boolean expressions, loops and asserts i.e
  • x.bar if isinstance(x, A) else ... now works
  • isinstance(x, A) and x.bar now works (shortcut expression, left and ensures type is valid in the right part)
  • isinstance(x, A) or x.foo now works (shortcut expression, left or ensures type is invalid in the right part)
  • while isinstance(x, A): print(x.bar) now works
  • assert isinstance(x, A); print(x.bar) now works
  • if isinstance(x, A): ... else: x.foo now works where x is cast to inverted in the else branch
• isinstance now works in conjunction with other terms as long as typecasting is guaranteed i.e. isinstance(x, A) and isinstance(y, B) and x.foo == y.bar now works
• isinstance(x, A) or isinstance(x, B) now casts x to Union[A, B]. note that isinstance(x, A) or isinstance(x, B) or whatever does not cast to anything since no type is actually guaranteed.
• not isinstance(x, A) now casts Union[A, B, ...] to Union[B, ...]
• the combination of and, or and not work in arbitrary and recursive combination

Note: This introduces a breaking change

While the below worked previously, this will break from this change onwards. The reason is that x is casted to "whatever union does not contain A" implicitly in the else branch and this can not be overwritten. Pulling in opinions on whether this will break real-life usecases from @juliusfrost @chrissiwaffler

if isinstance(x, A):
  ...
else:
  x = A(foo)

[juliusfrost]

I tested this in https://github.com/OpShin/opshin-pioneer-program with no issues

[chrissiwaffler]

No, I also don't see conflicts in any of my code.

Appreciate the changes! @nielstron
This will make things a lot easier with typecasting :)