drop hash allocations and optimize for success cases

We rarely care what the conflicts on a spec are, just whether or not
there are conflicts.  This commit introduces an allocation-free method
that returns whether or not there are conflicts.  The only case we
actually need to know the conflicts are in the case that an exception is
raised, and in that case we can do the work twice since the program will
exit anyway.  This drops hash allocations.

Benchmark:

```ruby
require 'stackprof'
require 'allocation_tracer'
require 'rubygems/test_case'
require 'rubygems/ext'
require 'rubygems/specification'

class TestGemSpecification < Gem::TestCase
  def test_find_in_unresolved_tree_is_not_exponentiental
    save_loaded_features do
      num_of_pkg = 7
      num_of_version_per_pkg = 3
      packages = (0..num_of_pkg).map do |pkgi|
        (0..num_of_version_per_pkg).map do |pkg_version|
          deps = Hash[(pkgi..num_of_pkg).map { |deppkgi| ["pkg#{deppkgi}", ">= 0"] }]
          new_spec "pkg#{pkgi}", pkg_version.to_s, deps
        end
      end
      base = new_spec "pkg_base", "1", {"pkg0" => ">= 0"}

      Gem::Specification.reset
      install_specs base,*packages.flatten
      base.activate

      require 'benchmark'
      ObjectSpace::AllocationTracer.setup(%i{path line type})
      r = ObjectSpace::AllocationTracer.trace do
        assert_raises(LoadError) { require 'no_such_file_foo' }
      end
      r.sort_by { |k,v| v.first }.each do |k,v|
        p k => v
      end
      p hash_alloc: ObjectSpace::AllocationTracer.allocated_count_table[:T_HASH]
      p :TOTAL => ObjectSpace::AllocationTracer.allocated_count_table.values.inject(:+)
    end
  end
end
```

Before: 1562496 Hash allocations
After: 0 Hash allocations

lib/rubygems/core_ext/kernel_require.rb

@@ -105,7 +105,7 @@ def require path
 
       # Ok, now find a gem that has no conflicts, starting
       # at the highest version.
-      valid = found_specs.select { |s| s.conflicts.empty? }.last
+      valid = found_specs.reject { |s| s.has_conflicts? }.last
 
       unless valid then
         le = Gem::LoadError.new "unable to find a version of '#{names.first}' to activate"

lib/rubygems/specification.rb

@@ -963,7 +963,7 @@ def self.find_in_unresolved_tree path
     specs.reverse_each do |spec|
       trails = []
       spec.traverse do |from_spec, dep, to_spec, trail|
-        next unless to_spec.conflicts.empty?
+        next if to_spec.has_conflicts?
         trails << trail if to_spec.contains_requirable_file? path
       end
 
@@ -1563,6 +1563,16 @@ def conflicts
     conflicts
   end
 
+  ##
+  # Return true if there are possible conflicts against the currently loaded specs.
+
+  def has_conflicts?
+    self.runtime_dependencies.any? { |dep|
+      spec = Gem.loaded_specs[dep.name]
+      spec and not spec.satisfies_requirement? dep
+    }
+  end
+
   ##
   # The date this gem was created.  Lazily defaults to the current UTC date.
   #
@@ -2154,10 +2164,8 @@ def check_version_conflict other # :nodoc:
   # Check the spec for possible conflicts and freak out if there are any.
 
   def raise_if_conflicts # :nodoc:
-    conf = self.conflicts
-
-    unless conf.empty? then
-      raise Gem::ConflictError.new self, conf
+    if has_conflicts? then
+      raise Gem::ConflictError.new self, conflicts
     end
   end