Removes unnecessary object allocations because of delegates

I introduced several singletons to store delegate instances to avoid unnecessary object allocation in generic methods. 

#### For example 
`ParallelEnumerable.PerformAggregation()` method 
__Before:__
```csharp
private static T PerformAggregation<T>(this ParallelQuery<T> source,
    Func<T, T, T> reduce, T seed, bool seedIsSpecified, bool throwIfEmpty, QueryAggregationOptions options)
{
    Contract.Assert(source != null);
    Contract.Assert(reduce != null);
    Contract.Assert(options.IsValidQueryAggregationOption(), "enum is out of range");

    AssociativeAggregationOperator<T, T, T> op = new AssociativeAggregationOperator<T, T, T>(
        source, seed, null, seedIsSpecified, reduce, reduce, delegate (T obj) { return obj; }, throwIfEmpty, options);
    return op.Aggregate();
}
```
```
	IL_0000: nop
	IL_0001: ldarg.0
	IL_0002: ldarg.2
	IL_0003: ldnull
	IL_0004: ldarg.3
	IL_0005: ldarg.1
	IL_0006: ldarg.1
	IL_0007: ldnull
	IL_0008: ldftn !!0 System.Linq.ParallelEnumerable::'<PerformAggregation>b__e'<!!T>(!!0)
>>> IL_000e: newobj instance void class [System.Runtime]System.Func`2<!!T, !!T>::.ctor(object, native int)
	IL_0013: ldarg.s throwIfEmpty
	IL_0015: ldarg.s options
	IL_0017: newobj instance void class System.Linq.Parallel.AssociativeAggregationOperator`3<!!T, !!T, !!T>::.ctor(class [System.Runtime]System.Collections.Generic.IEnumerable`1<!0>, !1, class [System.Runtime]System.Func`1<!1>, bool, class [System.Runtime]System.Func`3<!1, !0, !1>, class [System.Runtime]System.Func`3<!1, !1, !1>, class [System.Runtime]System.Func`2<!1, !2>, bool, valuetype System.Linq.Parallel.QueryAggregationOptions)
	IL_001c: stloc.0
	IL_001d: ldloc.0
	IL_001e: callvirt instance !2 class System.Linq.Parallel.AssociativeAggregationOperator`3<!!T, !!T, !!T>::Aggregate()
	IL_0023: stloc.1
	IL_0024: br.s IL_0026

	IL_0026: ldloc.1
	IL_0027: ret
```
__After:__

```csharp
private static T PerformAggregation<T>(this ParallelQuery<T> source,
    Func<T, T, T> reduce, T seed, bool seedIsSpecified, bool throwIfEmpty, QueryAggregationOptions options)
{
    Contract.Assert(source != null);
    Contract.Assert(reduce != null);
    Contract.Assert(options.IsValidQueryAggregationOption(), "enum is out of range");

    AssociativeAggregationOperator<T, T, T> op = new AssociativeAggregationOperator<T, T, T>(
        source, seed, null, seedIsSpecified, reduce, reduce, IdentityFunction<T>.Instance, throwIfEmpty, options);
    return op.Aggregate();
}
```

```
	IL_0000: nop
	IL_0001: ldarg.0
	IL_0002: ldarg.2
	IL_0003: ldnull
	IL_0004: ldarg.3
	IL_0005: ldarg.1
	IL_0006: ldarg.1
>>> IL_0007: ldsfld class [System.Runtime]System.Func`2<!0, !0> class System.Linq.IdentityFunction`1<!!T>::Instance
	IL_000c: ldarg.s throwIfEmpty
	IL_000e: ldarg.s options
	IL_0010: newobj instance void class System.Linq.Parallel.AssociativeAggregationOperator`3<!!T, !!T, !!T>::.ctor(class [System.Runtime]System.Collections.Generic.IEnumerable`1<!0>, !1, class [System.Runtime]System.Func`1<!1>, bool, class [System.Runtime]System.Func`3<!1, !0, !1>, class [System.Runtime]System.Func`3<!1, !1, !1>, class [System.Runtime]System.Func`2<!1, !2>, bool, valuetype System.Linq.Parallel.QueryAggregationOptions)
	IL_0015: stloc.0
	IL_0016: ldloc.0
	IL_0017: callvirt instance !2 class System.Linq.Parallel.AssociativeAggregationOperator`3<!!T, !!T, !!T>::Aggregate()
	IL_001c: stloc.1
	IL_001d: br.s IL_001f

	IL_001f: ldloc.1
	IL_0020: ret

```

Where `IdentityFunction` looks like
```csharp
internal class IdentityFunction<T>
{
    public static readonly Func<T, T> Instance = Function;

    private static T Function(T arg)
    {
        return arg;
    }
}
```