Alternate names for `List.concat` and `List.append`

This follows on from ideas in:

Comparison across languages

(m) means "mutates in place"

Language	Type	`x :: xs`	`xs ++ [x]`	`xs ++ ys`	[xs] -> xss
Lean	`List`	`cons x xs`	`concat xs x`	`append xs ys`	`Lean.join`
Lean	`Fin n -> X`	`cons x xs`	`snoc xs x`	`append xs ys`
Python	`list`	`xs.insert(0, x)` (m)	`xs.append(x)` (m)	`operator.concat(xs, ys)` (though `append` and `extend` also work)
Python	`deque`	`xs.appendleft(x)` (m)	`xs.appendright(x)` (m)	`operator.concat(xs, ys)`
PHP	`Array`	`array_unshift()`	`array_push()`	`array_merge()`
Cryptol	`Sequence` (size-dependent)	`[x] # xs`	`xs # [x]`	`(#)` (doc: "append").	`join`
Haskell	List	`(:)`	`snoc`	`(++)` (doc: "append")	`concat`