_type_lb and _type_ub are wrong

[cjdoris]

e.g. _type_ub(Vector{Vector{T}} where {T<:Int}) == Vector{Vector{Int}}, when the intention is that it is Vector{Vector{T} where {T<:Int}}

[cjdoris]

Actually the above comment is incorrect, because Vector{Vector{T} where {T<:Int}} is not a subtype of Vector{Vector{T}}.

I'm keeping _type_ub as it is, replacing each typevar with its upper bound. This gives the most general specialisation of the type.

_type_lb is similar, but any typevars which are a parameter of the outermost type are replaces with their lower bound.

e.g. _type_lb(Vector{Vector{T}} where {T<:Int}) == Vector{Vector{Int}} == _type_ub(...) because T is not a parameter of the top type.

e.g. _type_lb(Vector{<:Vector{<:Int}}) == Vector{Union{}} and _type_ub(Vector{<:Vector{<:Int}}) == Vector{Vector{<:Int}}