encoding/binary: fix error message

In the process, simplified internal sizeOf and
dataSize functions. Minor positive impact on
performance. Added test case.

benchmark                         old ns/op     new ns/op     delta
BenchmarkReadSlice1000Int32s      14006         14122         +0.83%
BenchmarkReadStruct               2508          2447          -2.43%
BenchmarkReadInts                 921           928           +0.76%
BenchmarkWriteInts                2086          2081          -0.24%
BenchmarkWriteSlice1000Int32s     13440         13497         +0.42%
BenchmarkPutUvarint32             28.5          26.3          -7.72%
BenchmarkPutUvarint64             81.3          76.7          -5.66%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkReadSlice1000Int32s      285.58       283.24       0.99x
BenchmarkReadStruct               27.90        28.60        1.03x
BenchmarkReadInts                 32.57        32.31        0.99x
BenchmarkWriteInts                14.38        14.41        1.00x
BenchmarkWriteSlice1000Int32s     297.60       296.36       1.00x
BenchmarkPutUvarint32             140.55       151.92       1.08x
BenchmarkPutUvarint64             98.36        104.33       1.06x

Fixes issue 6818.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/149290045