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`
Lean	`List`	`cons x xs`	`concat xs x`	`append xs ys`
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)`
Python	`deque`	`xs.appendleft(x)` (m)	`xs.appendright(x)` (m)	`operator.concat(xs, ys)`