Add a convert method for (module, trait, geom) (#194)

This allows the trait to be passed through completely from the time of discovery to the bottom level, which decreases allocations and dynamic dispatch in long call chains like GeometryOps' `apply`.

Author: asinghvi17

src/fallbacks.jl

@@ -148,6 +148,9 @@ calc_extent(t::AbstractFeatureCollectionTrait, fc) = reduce(Extents.union, filte
 # corresponding geometry type
 function convert(package::Module, geom)
     t = trait(geom)
+    convert(package, t, geom)
+end
+function convert(package::Module, t::AbstractTrait, geom)
     isdefined(package, :geointerface_geomtype) || throw(ArgumentError("$package does not implement `geointerface_geomtype`. Please request this be implemented in a github issue."))
     convert(package.geointerface_geomtype(t), t, geom)
 end

src/interface.jl

@@ -673,6 +673,21 @@ geom type for the trait using the modules `geointerface_traittype` method.
 
 `convert(T::Type)` or `convert(m::Module)` return curried versions of that function, 
 just like `==`.
+
+# Extended help
+
+For developers, the base implementation is `GeoInterface.convert(::YourGeomType, ::AbstractTrait, some_other_geom)`.
+This is the method signature that must be implemented for a custom geometry type.  
+    
+Users may also call this method directly, if they know the exact output - this can sometimes 
+save the Julia compiler some work and cause more optimal code to be generated.
+
+For convenience and type stability there is a `convert(::Module, ::AbstractTrait, geom)` method
+that is particularly useful when the trait of a geometry needs to be propagated deep within the 
+stack.  
+
+This helps to prevent the compiler from losing the trait information, especially useful 
+when dealing with sumtype-like implementations or opaque pointers from C libraries.
 """
 convert(T, geom) = convert(T, geomtrait(geom), geom)
 convert(::Type{T}, x::T) where {T} = x  # no-op