single `Jl` type for Julia values

[cjdoris]

Make symmetric with Py:

• Operations with Jl all return Jl.
• Arguments to methods of Jl convert args using pyconvert(Any, x).
• Jl(x) to convert Python x to Julia object.
• Wrapper types JlArray, JlDict, JlIO etc.
• __julia_value__() method signifies wrapper types (similar to ispy(x) and Py(x) for Python wrapper types)?