Allow non-string indices for Trie

[jonas-schulze]

by dropping the ::AbstractString restriction. I considered replacing that by ::Union{AbstractString,AbstractVector} (thereby wondering why AbstractString <: AbstractVector doesn't hold). Which one would you prefer?

This is breaking as the original type parameter (denoting the value type) is now in second position, such that the parameters follow the common key-value pattern like e.g. Dict has. I noticed that you are feature-frozen (#479), but I think it would be better to consider changing the order now instead of adding this feature later in a non-breaking manner (i.e. reversing the type parameters).

This is ready for comments. If there is anything I can change to improve this PR, I'm happy to do so. 🙂

[oxinabox]

This seems sensible.
I will try and find time to review properly later this week

[oxinabox]

Is all we need to add in order to support arbitrary iterators just:

Suggested change

	_concat(prefix::Vector{T}, char::T) where {T} = vcat(prefix, char)
	_concat(prefix, char) = Iterators.flatten(prefix, (char,))

?

[jonas-schulze]

Do you mean Iterators.flatten((prefix, (char,)))?
That would cause the types to explode:

julia> p1 = [1]
1-element Vector{Int64}:
 1

julia> p2 = Iterators.flatten((p1, (2,)))
Base.Iterators.Flatten{Tuple{Vector{Int64}, Tuple{Int64}}}(([1], (2,)))

julia> p3 = Iterators.flatten((p2, (3,)))
Base.Iterators.Flatten{Tuple{Base.Iterators.Flatten{Tuple{Vector{Int64}, Tuple{Int64}}}, Tuple{Int64}}}((Base.Iterators.Flatten{Tuple{Vector{Int64}, Tuple{Int64}}}(([1], (2,))), (3,)))

As _concat only receives objects derived from _empty_prefix, which are either Strings or Vectors, I don't think this needs to be generalized further. Instead, it might be good to wrap the 3-argument keys (Base.keys(t) = _keys(t)), such that arguments 2 and 3 are not accessible to the average user.

[oxinabox]

This is a really good change and well written.

I will wait a bit re: my question about arbitrary iterators before merge,
but this basically looks great