Joins two lists of associations by matching values of a key.
JoinAcross[{a1, a2, …}, {b1, b2, …}, keyspec]
JoinAcross[tab1, tab2, keyspec]
JoinAcross[prefix1 -> obj1, prefix2 -> obj2, keyspec]
JoinAcross[alist, blist, keyspec, "joinspec"]
JoinAcross[{<|"a" -> 1|>}, {<|"a" -> 1, "b" -> 2|>}, "a"]
→ {<|a -> 1, b -> 2|>}