Add cast!() macro

[skejserjensen]

PR #275 showed that casts from Any to a concrete type is sometimes needed outside of extracting arrays from Vecs and Recordbatches. Thus, this PR adds the cast!() macro which is defined as $expr.as_any().downcast_ref::<$type>().unwrap() and implements the value!() and array!() macros in terms of it. The PR also replaces all uses of as_any().downcast_ref().unwrap() found by ripgrep with the macro.