Make validateToken a function

[davecheney]

Now the byteReader change is landed, validateToken doesn't need access
to the scanner any more. Rewrite it to take a *byteReader as a faux
receiver. It turns out this is ~1% faster overall, and I prefer the
purity of moving this from a function to a method. If the trend holds
true for parseString and parseNumber, this will make it easier to
refactor the latter.

name                             old time/op    new time/op    delta
Scanner/canada.json.gz-16          3.56ms ± 1%    3.50ms ± 0%  -1.65%  (p=0.008 n=5+5)
Scanner/citm_catalog.json.gz-16    1.57ms ± 1%    1.56ms ± 1%  -1.02%  (p=0.008 n=5+5)
Scanner/twitter.json.gz-16          849µs ± 1%     833µs ± 1%  -1.85%  (p=0.008 n=5+5)
Scanner/code.json.gz-16            3.69ms ± 1%    3.62ms ± 1%  -1.96%  (p=0.008 n=5+5)
Scanner/example.json.gz-16         16.8µs ± 1%    16.6µs ± 1%  -1.25%  (p=0.016 n=5+5)
Scanner/sample.json.gz-16           476µs ± 0%     468µs ± 0%  -1.69%  (p=0.008 n=5+5)

name                             old speed      new speed      delta
Scanner/canada.json.gz-16         632MB/s ± 1%   642MB/s ± 0%  +1.68%  (p=0.008 n=5+5)
Scanner/citm_catalog.json.gz-16  1.10GB/s ± 1%  1.11GB/s ± 1%  +1.02%  (p=0.008 n=5+5)
Scanner/twitter.json.gz-16        744MB/s ± 1%   758MB/s ± 1%  +1.89%  (p=0.008 n=5+5)
Scanner/code.json.gz-16           525MB/s ± 1%   536MB/s ± 1%  +2.00%  (p=0.008 n=5+5)
Scanner/example.json.gz-16        775MB/s ± 1%   784MB/s ± 1%  +1.26%  (p=0.016 n=5+5)
Scanner/sample.json.gz-16        1.44GB/s ± 0%  1.47GB/s ± 0%  +1.72%  (p=0.008 n=5+5)

name                             old alloc/op   new alloc/op   delta
Scanner/canada.json.gz-16           0.00B          0.00B         ~     (all equal)
Scanner/citm_catalog.json.gz-16     0.00B          0.00B         ~     (all equal)
Scanner/twitter.json.gz-16          0.00B          0.00B         ~     (all equal)
Scanner/code.json.gz-16             0.00B          0.00B         ~     (all equal)
Scanner/example.json.gz-16          0.00B          0.00B         ~     (all equal)
Scanner/sample.json.gz-16           0.00B          0.00B         ~     (all equal)

name                             old allocs/op  new allocs/op  delta
Scanner/canada.json.gz-16            0.00           0.00         ~     (all equal)
Scanner/citm_catalog.json.gz-16      0.00           0.00         ~     (all equal)
Scanner/twitter.json.gz-16           0.00           0.00         ~     (all equal)
Scanner/code.json.gz-16              0.00           0.00         ~     (all equal)
Scanner/example.json.gz-16           0.00           0.00         ~     (all equal)
Scanner/sample.json.gz-16            0.00           0.00         ~     (all equal)