graphviz: tweak data structure slightly

A node's edges contain an associative array of target nodes rather than
a linear array.

This way we automatically dedup edges when we load multiple graphs.

graphviz.tlv

@@ -738,10 +738,9 @@
  > -- edge_stmt
  > local tok3 = tokens:read()
  > if graph[tok1] == nil then
- > graph[tok1] = {tok3}
- > else
- > append(graph[tok1], {tok3})
+ > graph[tok1] = {}
  > end
+ > graph[tok1][tok3] = true
  > elseif tok2 == '--' then
  > error('unexpected token "--" in digraph; edges should be directed using "->"')
  > elseif tok2 == '=' then
@@ -808,7 +807,7 @@
  >function sources(Graph)
  > local is_target = {}
  > for source, targets in pairs(Graph) do
- > for _, target in ipairs(targets) do
+ > for target, _ in pairs(targets) do
  > is_target[target] = true
  > end
  > end