tun: use add-with-carry in checksumNoFold()

tun: use add-with-carry in checksumNoFold()

Use parallel summation with native byte order per RFC 1071.
add-with-carry operation is used to add 4 words per operation.  Byteswap
is performed before and after checksumming for compatibility with old
`checksumNoFold()`.  With this we get a 30-80% speedup in `checksum()`
depending on packet sizes.

Add unit tests with comparison to a per-word implementation.

**Intel(R) Xeon(R) Silver 4210R CPU @ 2.40GHz**

| Size | OldTime | NewTime | Speedup  |
|------|---------|---------|----------|
| 64   | 12.64   | 9.183   | 1.376456 |
| 128  | 18.52   | 12.72   | 1.455975 |
| 256  | 31.01   | 18.13   | 1.710425 |
| 512  | 54.46   | 29.03   | 1.87599  |
| 1024 | 102     | 52.2    | 1.954023 |
| 1500 | 146.8   | 81.36   | 1.804326 |
| 2048 | 196.9   | 102.5   | 1.920976 |
| 4096 | 389.8   | 200.8   | 1.941235 |
| 8192 | 767.3   | 413.3   | 1.856521 |
| 9000 | 851.7   | 448.8   | 1.897727 |
| 9001 | 854.8   | 451.9   | 1.891569 |

**AMD EPYC 7352 24-Core Processor**

| Size | OldTime | NewTime | Speedup  |
|------|---------|---------|----------|
| 64   | 9.159   | 6.949   | 1.318031 |
| 128  | 13.59   | 10.59   | 1.283286 |
| 256  | 22.37   | 14.91   | 1.500335 |
| 512  | 41.42   | 24.22   | 1.710157 |
| 1024 | 81.59   | 45.05   | 1.811099 |
| 1500 | 120.4   | 68.35   | 1.761522 |
| 2048 | 162.8   | 90.14   | 1.806079 |
| 4096 | 321.4   | 180.3   | 1.782585 |
| 8192 | 650.4   | 360.8   | 1.802661 |
| 9000 | 706.3   | 398.1   | 1.774177 |
| 9001 | 712.4   | 398.2   | 1.789051 |

Signed-off-by: Tu Dinh Ngoc <dinhngoc.tu@irit.fr>
[Jason: simplified and cleaned up unit tests]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>