allow for non-ASCII names when creating an `AbstractSublevel`

[dcrousso]

would it be possible to add an option when creating an AbstractSublevel (or just remove the requirement altogether) to allow for names with non-ASCII characters?

it seems a bit odd that i could manually create keys for put/get/del/etc. that contain non-ASCII characters, but i cant use sublevel (which is a far nicer experience due to the behavior of keys/iterator/etc.)

for example,

level.get("!foo!bar")

vs

let sublevel = level.sublevel("foo")
sublevel.get("bar")