Enable three-argument expect with message

[seancorfield]

• (expect a) is supported like (is a)
• (expect e a) is supported like (is (= e a)) (and its variants)
• (expect e a msg) is not supported, so there's not equivalent to (is expr msg)

This would require pushing ex? and e' one argument further -- or breaking expect into two macros, one documented, one not, which might be better...?