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

[dinhngtu]

Reopened with a fix for initial values.

This PR proposes a speedup for 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 and the old big-endian 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