Prettify type printing in signature help/hover/...

[fredrikekre]

For example

struct Foo
    x::Vector{Int}
    y::Union{Int,Nothing}
end

shows up as

struct Foo <: Core.Any
    x::Core.Array{Core.Int64, 1}
    y::Union{Core.Int64,Core.Nothing}
end

It seems that:

1. <: Core.Any can be dropped
2. Core. can be dropped (if exported?)
3. Array{T,1} and Array{T,2} can be displayed as Vector{T} and Matrix{T}.