`private` parameter for `hasproperty`

[jakobjpeters]

hasproperty is implemented as:

hasproperty(x, s::Symbol) = s in propertynames(x)

If one wants to check hasproperty for a private name (such as a non-public name in a module), they currently need to do s in propertynames(x, true). This change would allow hasproperty(x, s, true) instead, which is otherwise unintuitive until one looks at the implementation for hasproperty.

[jakobjpeters]

I guess another change is needed for modules:

propertynames(x) = fieldnames(typeof(x))
propertynames(m::Module) = names(m)
propertynames(x, private::Bool) = propertynames(x) # ignore private flag by default
propertynames(x::Array) = () # hide the fields from tab completion to discourage calling `x.size` instead of `size(x)`, even though they are equivalent

This needs a new method

propertynames(m::Module, private) = names(m; all = private)