Eliminated unnecessary string conversions throughout and removed
(internal) capability for arbitrary character sets in conversion
routines (functionality was not exported and not used internally).

benchmark                         old ns/op     new ns/op     delta
BenchmarkDecimalConversion-8      198283        187085        -5.65%
BenchmarkStringPiParallel-8       46116         47822         +3.70%
BenchmarkString10Base2-8          216           166           -23.15%
BenchmarkString100Base2-8         886           762           -14.00%
BenchmarkString1000Base2-8        7296          6625          -9.20%
BenchmarkString10000Base2-8       72371         65563         -9.41%
BenchmarkString100000Base2-8      725849        672766        -7.31%
BenchmarkString10Base8-8          160           114           -28.75%
BenchmarkString100Base8-8         398           309           -22.36%
BenchmarkString1000Base8-8        2650          2244          -15.32%
BenchmarkString10000Base8-8       24974         21745         -12.93%
BenchmarkString100000Base8-8      245457        217489        -11.39%
BenchmarkString10Base10-8         337           288           -14.54%
BenchmarkString100Base10-8        1298          1046          -19.41%
BenchmarkString1000Base10-8       6200          5752          -7.23%
BenchmarkString10000Base10-8      24942         22589         -9.43%
BenchmarkString100000Base10-8     8012921       7947152       -0.82%
BenchmarkString10Base16-8         156           107           -31.41%
BenchmarkString100Base16-8        344           255           -25.87%
BenchmarkString1000Base16-8       2067          1705          -17.51%
BenchmarkString10000Base16-8      19026         16112         -15.32%
BenchmarkString100000Base16-8     184038        163457        -11.18%

Change-Id: I68bd807529bd9b985f4b6ac2a87764bcc1a7d2f7
Reviewed-on: https://go-review.googlesource.com/14926Reviewed-by: Alan Donovan <adonovan@google.com>