Issues deconstructing optional structs

[garborg]

Whereas #16 deals w/ constructing optional structs, here are a couple issues deconstructing them:

using Arrow: Arrow
using Strapping: Strapping
using StructTypes: StructTypes

# Types

# N.B. Making `TInterval` an alias for `Int` avoids both errors below
# const TInterval = Int
struct TInterval
    a::Int
end
StructTypes.StructType(::Type{TInterval}) = StructTypes.Struct()

struct StopLoc
    b::Int
    # N.B. Missing has the same issues as Nothing
    hours::Union{Nothing,TInterval}
end
StructTypes.StructType(::Type{StopLoc}) = StructTypes.Struct()

struct Trip
    start::StopLoc
    finish::StopLoc
end
StructTypes.StructType(::Type{Trip}) = StructTypes.Struct()

struct TripEnd
    finish::StopLoc
end
TripEnd(t::Trip) = TripEnd(t.finish)
StructTypes.StructType(::Type{TripEnd}) = StructTypes.Struct()

# Generate data

trips = [
    Trip(StopLoc(1, nothing), StopLoc(2, nothing)),
    Trip(StopLoc(2, TInterval(1)), StopLoc(3, TInterval(2))),
]

trip_ends = TripEnd.(trips)

# Deconstruct

de_trips = Strapping.deconstruct(trips)
Arrow.write("de_trips.arrow", de_trips) # => ERROR:
# LoadError: duplicate field name in NamedTuple: "finish_lng" is not unique

de_trip_ends = Strapping.deconstruct(trip_ends)
Arrow.write("de_trip_ends.arrow", de_trip_ends) # => ERROR:
# LoadError: AssertionError: fn === nothing