Use concrete value for quad.Quad

Comparison of -short benchmarks in cayley.

$ benchcmp pointer.bench concrete.bench
benchmark                                   old ns/op     new ns/op	delta
BenchmarkNamePredicate                      1673276       1655093	-1.09%
BenchmarkLargeSetsNoIntersection            318985907     261499984	-18.02%
BenchmarkNetAndSpeed                        104403743     41516981	-60.23%
BenchmarkKeanuAndNet                        17309258      16857513	-2.61%
BenchmarkKeanuAndSpeed                      20159161      19282833	-4.35%

Comparison of pathological cases are not so happy.

benchmark                                   old ns/op       new ns/op		delta
BenchmarkVeryLargeSetsSmallIntersection     55269775527     246084606672	+345.24%
BenchmarkHelplessContainsChecker            23436501319     24308906949		+3.72%

Profiling the worst case:

Pointer:
Total: 6121 samples
    1973  32.2%  32.2%     1973  32.2% runtime.findfunc
     773  12.6%  44.9%      773  12.6% readvarint
     510   8.3%  53.2%      511   8.3% step
     409   6.7%  59.9%      410   6.7% runtime.gentraceback
     390   6.4%  66.2%      391   6.4% pcvalue
     215   3.5%  69.8%      215   3.5% runtime.funcdata
     181   3.0%  72.7%      181   3.0% checkframecopy
     118   1.9%  74.6%      119   1.9% runtime.funcspdelta
      96   1.6%  76.2%       96   1.6% runtime.topofstack
      76   1.2%  77.5%       76   1.2% scanblock

Concrete:
Total: 25027 samples
    9437  37.7%  37.7%     9437  37.7% runtime.findfunc
    3853  15.4%  53.1%     3853  15.4% readvarint
    2366   9.5%  62.6%     2366   9.5% step
    2186   8.7%  71.3%     2186   8.7% runtime.gentraceback
    1816   7.3%  78.5%     1816   7.3% pcvalue
    1016   4.1%  82.6%     1016   4.1% runtime.funcdata
     859   3.4%  86.0%      859   3.4% checkframecopy
     506   2.0%  88.1%      506   2.0% runtime.funcspdelta
     410   1.6%  89.7%      410   1.6% runtime.topofstack
     303   1.2%  90.9%      303   1.2% runtime.newstack

graph/triplestore.go

@@ -40,17 +40,17 @@ type Value interface{}
 
 type TripleStore interface {
 	// Add a triple to the store.
-	AddTriple(*quad.Quad)
+	AddTriple(quad.Quad)
 
 	// Add a set of triples to the store, atomically if possible.
-	AddTripleSet([]*quad.Quad)
+	AddTripleSet([]quad.Quad)
 
 	// Removes a triple matching the given one  from the database,
 	// if it exists. Does nothing otherwise.
-	RemoveTriple(*quad.Quad)
+	RemoveTriple(quad.Quad)
 
 	// Given an opaque token, returns the triple for that token from the store.
-	Quad(Value) *quad.Quad
+	Quad(Value) quad.Quad
 
 	// Given a direction and a token, creates an iterator of links which have
 	// that node token in that directional field.