Rename `Bwd.append` to `Bwd.append_list`

[TOTBWF]

Currently, Bwd.append has type 'a bwd -> 'a list -> 'a bwd; this isn't consistent with the API for lists, which has a homogenous List.append : 'a list -> 'a list -> 'a list. I propose that we rename the current Bwd.append to Bwd.append_list, Bwd.prepend to Bwd.prepend_list, and add a homogenous Bwd.append : 'a bwd -> 'a bwd -> 'a bwd function.

[favonia]

I think it's possible to justify the current usage of append, but I would like to talk about something else first: I'm a bit hesitant about prepand_list because it is not prepanding a list. Could you possibly elaborate on it?

[TOTBWF]

I mostly chose the name for the sake of consistency; I suppose we could call it prepend_rev or prepend_bwd instead.