Implement if-case

[egranata]

Fixes #84

[egranata]

This change does not compose well, for example one cannot do

val maybe = Maybe::Some(5);

if case Some(n) = maybe && n > 3 { ... }

or even just if maybe isa Maybe and case Some(n) = maybe { ... }

so this syntax seems more ergonomic but it composes poorly, i.e. not at all, is prone to raising runtime errors (e.g. if case Foo = 3) without a clean simple syntax to avoid that, and it also does not compose well with multiple values, e.g.

val a = Result::Ok(3);
val b = Maybe::Some(4);

if case Ok(val_a) = a and case Some(val_b) = b { ... }

to be fair, match also does not do a good job of the latter as there is no way to match on a list of elements and pattern match only some/all of them to something

The long and short of it is, this syntax is not a simple composable block, but rather some ad-hoc piece of machinery, and it is not obvious to me that it carries its weight