RFC: updates for changes to Julia

This doesn't work yet, but it goes part of the way towards updating Debug against recent Julia.

I'm a bit stumped by this one, however:
```
julia> using Debug
ERROR: Ex not defined
 in include_from_node1 at loading.jl:76
 in reload_path at loading.jl:96
 in require at loading.jl:48
 in include_from_node1 at loading.jl:76
 in reload_path at loading.jl:96
 in require at loading.jl:48
at /home/tim/.julia/Debug/src/Analysis.jl:15
at /home/tim/.julia/Debug/src/Debug.jl:7

julia> Meta.Ex
Union(Expr,Node{ExValue})

julia> Ex
ERROR: Ex not defined

julia> using Meta

julia> Ex
Union(Expr,Node{ExValue})
```
Seems odd given that Analysis is saying `using Meta`. I'm wondering if it's a Julia bug?