move the tiny helper types to their own source file

docs/src/index.md

@@ -59,6 +59,13 @@ julia> Matrix(S)
 
 # Function reference
 
+## Helper types
+
+```@autodocs
+Modules = [SBML]
+Pages = ["types.jl"]
+```
+
 ## Data structures
 
 ```@autodocs

src/SBML.jl

@@ -4,14 +4,15 @@ using SBML_jll, Libdl
 using SparseArrays
 using Symbolics
 
+include("types.jl")
 include("structs.jl")
 include("version.jl")
 
-include("utils.jl")
-include("readsbml.jl")
 include("converters.jl")
 include("math.jl")
+include("readsbml.jl")
 include("symbolics.jl")
+include("utils.jl")
 
 sbml(sym::Symbol) = dlsym(SBML_jll.libsbml_handle, sym)
 

src/structs.jl

 
-"""
-    Maybe{X}
-
-Type shortcut for "`X` or nothing" or "nullable `X`" in javaspeak. Name
-got inspired by our functional friends.
-"""
-const Maybe{X} = Union{Nothing,X}
-
 """
 Part of a measurement unit definition that corresponds to the SBML definition
 of `Unit`. For example, the unit "per square megahour", Mh^(-2), is written as:

src/utils.jl

-const VPtr = Ptr{Cvoid}
-
 """
     function getS(m::SBML.Model; zeros=spzeros)::Tuple{Vector{String},Vector{String},AbstractMatrix{Float64}}