Consider turning the enums into types for dispatch

[yeesian]

See e.g. https://discourse.julialang.org/t/style-recommendation-for-enum-as-type/46078/5 and https://www.juliabloggers.com/julia-dispatching-enum-versus-type/ for a discussion.

This is helpful for e.g. dispatching on OGRFIELDTYPE in

ArchGDAL.jl/src/ogr/feature.jl

Lines 390 to 413 in 8f820c7

	const _FETCHFIELD = Dict{OGRFieldType,Function}(
	OFTInteger => asint,
	OFTIntegerList => asintlist,
	OFTReal => asdouble,
	OFTRealList => asdoublelist,
	OFTString => asstring,
	OFTStringList => asstringlist,
	OFTBinary => asbinary,
	OFTDate => asdatetime,
	OFTTime => asdatetime,
	OFTDateTime => asdatetime,
	OFTInteger64 => asint64,
	OFTInteger64List => asint64list,
	)
	function getfield(feature::Feature, i::Integer)
	return if isfieldset(feature, i)
	_fieldtype = gettype(getfielddefn(feature, i))
	_fetchfield = get(_FETCHFIELD, _fieldtype, getdefault)
	_fetchfield(feature, i)
	else
	getdefault(feature, i)
	end
	end