Use unique IDs for unresolved references

[Morriar]

Iterating from Ruby on unresolved references without the ID handle trick is painful, I think we need to create unique IDs for the references.

Take this simple script:

graph = Saturn::Graph.new
graph.index_all([path])

refs = graph.unresolved_references
  .grep(Saturn::UnresolvedConstantReference)
  .select { |ref| ref.name.include?("Shopify") }

puts "Selected: #{refs.size}"

Here's a comparison of performances between an array of references and an array of reference ids:

# array of references

Maximum Resident Set Size: 7427211264 bytes (7083.14 MB)
Peak Memory Footprint:     9216089600 bytes (8789.14 MB)
Execution Time:            1301.68 seconds

# array of ID handles

Maximum Resident Set Size: 2281750528 bytes (2176.04 MB)    x3.2 smaller
Peak Memory Footprint:     1567594496 bytes (1494.97 MB)    x5.8 smaller
Execution Time:            125.06 seconds                   x10.4 faster