add vendor

vendor/github.com/jtolds/gls/gen_sym.go

@@ -0,0 +1,13 @@
+package gls
+
+var (
+	symPool = &idPool{}
+)
+
+// ContextKey is a throwaway value you can use as a key to a ContextManager
+type ContextKey struct{ id uint }
+
+// GenSym will return a brand new, never-before-used ContextKey
+func GenSym() ContextKey {
+	return ContextKey{id: symPool.Acquire()}
+}