add `propertynames`

[ericphanson]

Closes #131

Here, I define the "public" properties as the getproperty overloads, and the private properties as any remaining internal fields. My thinking is then downstream types can accept this or define their own propertynames to declare which fields whey want to be private or not.

I used Base.merge_names here; this is a Julia internal function so perhaps we should not depend on it. It has at least existed for 4 years (https://github.com/JuliaLang/julia/blame/ff001a4e1f5b7f949f5a2328520a7e78bc1957b4/base/namedtuple.jl#L211). The reason I wanted to use it is to have a fast way of merging the tuples of symbols without duplicates (a slow alternative would be Tuple(unique((public_names..., fieldnames(T)...)))).

[nickrobinson251]

I used Base.merge_names here... to have a fast way of merging the tuples of symbols without duplicates

I think this is the same motivation as for the hack NamedDims.jl uses to make operations on tuples of symbols non-allocating https://github.com/invenia/NamedDims.jl/blob/77b09418820bca221d5bff68eacd613a8bfac6ed/src/name_core.jl#L10-L30 Anyway, using Base.merge_names seems fine to me. But then again, i suspect propertynames(x; private=true) being slow/allocating seems fine too.