merge now throws when a non-first arg is a scalar, a set, a list, or a
wrong-length vector (a length-2 vector or a map still merge). Other map-like
tables (records/sorted-maps/host tables, e.g. SCI's namespaces) keep the lenient
conj path so the SCI bootstrap still loads.
merge 29/11/2 -> 35/3/4. clojure-test-suite pass 3874->3880.
spec: 5 merge strictness cases. jpm test green.