Implement to_vec for DataTypes

[mzgubic]

I am surprised we didn't run into this earlier.

julia> v, b = to_vec(Float64)
ERROR: StackOverflowError:
Stacktrace:
     [1] fieldnames
       @ ./reflection.jl:177 [inlined]
     [2] to_vec(x::Type{Float64})
       @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/to_vec.jl:42
     [3] (::FiniteDifferences.var"#206#209"{Core.TypeName})(name::Symbol)
       @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/to_vec.jl:38
     [4] map (repeats 4 times)
       @ ./tuple.jl:216 [inlined]
     [5] to_vec(x::Core.TypeName)
       @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/to_vec.jl:54
     [6] (::FiniteDifferences.var"#206#209"{DataType})(name::Symbol)
       @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/to_vec.jl:38
     [7] map(f::FiniteDifferences.var"#206#209"{DataType}, t::NTuple{20, Symbol})
       @ Base ./tuple.jl:226
     [8] to_vec(x::Type{Float64})
       @ FiniteDifferences ~/JuliaEnvs/test/alex/dev/FiniteDifferences/src/to_vec.jl:54--- the last 6 lines are repeated 3905 more times ---

Found since it caused problems with unflatten from ParameterHandling. cc @AlexRobson

[willtebbutt]

LGTM