math/big: permit internal nat.scan to accept decimal point
This will simplify parsing of rational and (eventually) floating point numbers.
Also streamlined inner loop. As a result, scan runs faster for all but short
(<= 10 digit) numbers. For short numbers it is < 10% slower (cause is known
and could be addressed in a future CL).
Minor unrelated cleanups. Added additional tests.
benchmark old ns/op new ns/op delta
BenchmarkScanPi 134465 125122 -6.95%
BenchmarkScan10Base2 493 540 +9.53%
BenchmarkScan100Base2 3608 3244 -10.09%
BenchmarkScan1000Base2 35376 32377 -8.48%
BenchmarkScan10000Base2 481504 450028 -6.54%
BenchmarkScan100000Base2
17936774 17648080 -1.61%
BenchmarkScan10Base8 258 280 +8.53%
BenchmarkScan100Base8 1389 1323 -4.75%
BenchmarkScan1000Base8 14221 13036 -8.33%
BenchmarkScan10000Base8 271298 258993 -4.54%
BenchmarkScan100000Base8
15715465 15672580 -0.27%
BenchmarkScan10Base10 249 268 +7.63%
BenchmarkScan100Base10 1324 1220 -7.85%
BenchmarkScan1000Base10 13398 12234 -8.69%
BenchmarkScan10000Base10 259157 249342 -3.79%
BenchmarkScan100000Base10
15670613 15582409 -0.56%
BenchmarkScan10Base16 231 251 +8.66%
BenchmarkScan100Base16 1093 1065 -2.56%
BenchmarkScan1000Base16 12687 12196 -3.87%
BenchmarkScan10000Base16 282349 271443 -3.86%
BenchmarkScan100000Base16
16742669 16552917 -1.13%
Change-Id: I4b9b078792788aef872b307399f00ffd34903127
Reviewed-on: https://go-review.googlesource.com/2960
Reviewed-by: Alan Donovan <adonovan@google.com>