🍛 Curry the API (more)

[favonia]

@jonsterling expressed that we should perhaps curry the API more aggressively. In particular, these four functions can be curried without causing confusion:

val include_singleton : Trie.path * (Param.data * Param.tag) -> unit
val include_subtree : Trie.path * (Param.data, Param.tag) Trie.t -> unit
val import_singleton : Trie.path * (Param.data * Param.tag) -> unit
val import_subtree : Trie.path * (Param.data, Param.tag) Trie.t -> unit