4.x: Improve the performance of Repeat()

[akarnokd]

This PR improves the performance of Repeat() as well as reduces the allocations in it.

The changes are similar to #684:

• Split the implementation into recursive and long-running variants
• Use the sink fields to keep track of numbers
• Use direct task management for the recursive scheduling parts

Benchmark comparison:

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i7-4790 CPU 3.60GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
Frequency=3513586 Hz, Resolution=284.6095 ns, Timer=TSC
  [Host]     : .NET Framework 4.6.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.3110.0
  DefaultJob : .NET Framework 4.6.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.3110.0

// =============================
// Before

          Method |       N |             Mean |           Error |           StdDev |      Gen 0 |   Allocated |
---------------- |-------- |-----------------:|----------------:|-----------------:|-----------:|------------:|
 Repeat_Infinite |       1 |       1,124.2 ns |        14.62 ns |        13.676 ns |     0.2289 |       968 B |
   Repeat_Finite |       1 |         658.9 ns |        10.03 ns |         8.888 ns |     0.1898 |       800 B |
 Repeat_Infinite |      10 |       4,746.5 ns |        44.52 ns |        41.648 ns |     0.5035 |      2144 B |
   Repeat_Finite |      10 |       4,015.5 ns |        19.13 ns |        15.978 ns |     0.4807 |      2032 B |
 Repeat_Infinite |     100 |      40,143.7 ns |       265.54 ns |       248.388 ns |     3.2959 |     13944 B |
   Repeat_Finite |     100 |      37,831.2 ns |       148.16 ns |       138.590 ns |     3.2959 |     13833 B |
 Repeat_Infinite |    1000 |     395,852.8 ns |     2,473.30 ns |     2,313.527 ns |    30.7617 |    130286 B |
   Repeat_Finite |    1000 |     374,635.1 ns |     3,186.28 ns |     2,980.447 ns |    30.7617 |    130174 B |
 Repeat_Infinite |   10000 |   3,958,720.3 ns |     9,353.88 ns |     7,302.893 ns |   304.6875 |   1290444 B |
   Repeat_Finite |   10000 |   3,791,888.7 ns |    13,149.28 ns |    10,980.241 ns |   304.6875 |   1290312 B |
 Repeat_Infinite |  100000 |  40,644,575.8 ns |   655,346.49 ns |   613,011.451 ns |  3062.5000 |  12899958 B |
   Repeat_Finite |  100000 |  37,461,446.1 ns |   172,926.03 ns |   161,755.103 ns |  3062.5000 |  12899840 B |
 Repeat_Infinite | 1000000 | 393,461,337.2 ns | 1,903,940.74 ns | 1,780,947.174 ns | 30750.0000 | 128990086 B |
   Repeat_Finite | 1000000 | 377,989,073.5 ns | 6,045,007.96 ns | 5,654,503.648 ns | 30750.0000 | 128989308 B |

// =============================
// After

          Method |       N |             Mean |            Error |           StdDev |      Gen 0 |  Allocated |
---------------- |-------- |-----------------:|-----------------:|-----------------:|-----------:|-----------:|
 Repeat_Infinite |       1 |         929.7 ns |        10.904 ns |         9.666 ns |     0.1554 |      656 B |
   Repeat_Finite |       1 |         575.5 ns |        10.578 ns |         9.895 ns |     0.1230 |      520 B |
 Repeat_Infinite |      10 |       2,959.2 ns |        12.433 ns |        11.629 ns |     0.2899 |     1232 B |
   Repeat_Finite |      10 |       2,266.2 ns |         6.947 ns |         6.498 ns |     0.2594 |     1096 B |
 Repeat_Infinite |     100 |      19,660.3 ns |        52.412 ns |        49.027 ns |     1.6479 |     6992 B |
   Repeat_Finite |     100 |      21,740.8 ns |       153.386 ns |       143.478 ns |     1.6174 |     6856 B |
 Repeat_Infinite |    1000 |     223,735.8 ns |       841.558 ns |       787.194 ns |    15.3809 |    64593 B |
   Repeat_Finite |    1000 |     187,399.5 ns |     1,283.175 ns |     1,200.282 ns |    15.1367 |    64457 B |
 Repeat_Infinite |   10000 |   1,885,503.8 ns |     6,665.867 ns |     6,235.255 ns |   152.3438 |   640605 B |
   Repeat_Finite |   10000 |   1,883,624.3 ns |     6,752.810 ns |     6,316.582 ns |   152.3438 |   640477 B |
 Repeat_Infinite |  100000 |  18,769,865.2 ns |    65,556.399 ns |    61,321.490 ns |  1500.0000 |  6400900 B |
   Repeat_Finite |  100000 |  18,871,466.9 ns |    99,828.389 ns |    93,379.528 ns |  1500.0000 |  6400644 B |
 Repeat_Infinite | 1000000 | 188,786,400.6 ns |   679,486.687 ns |   635,592.207 ns | 15250.0000 | 64001342 B |
   Repeat_Finite | 1000000 | 188,306,308.8 ns | 1,101,114.930 ns | 1,029,983.488 ns | 15250.0000 | 64001340 B |