Boxed math warning at unchecked-add

[valerauko]

The calls to unchecked-add in collections cause boxed math warnings when *unchecked-math* is enabled.

https://github.com/ztellman/potemkin/blob/e8c64802b8772a90110434d1b515ce35b78b961b/src/potemkin/collections.clj#L120

https://github.com/ztellman/potemkin/blob/e8c64802b8772a90110434d1b515ce35b78b961b/src/potemkin/collections.clj#L128

Type hinting acc as long resolves the warning.