encoding/json: optimize isValidNumber functionjson-isValidNumber

We use an index instead of reslicing the string because reslicing a string
updates both the data and length fields instead of just the one index
variable.

Benchmark result,

name             old time/op    new time/op    delta
NumberIsValid-8  19.0ns ± 0%    14.5ns ± 1%  -23.70%  (p=0.008 n=5+5)

name             old alloc/op   new alloc/op   delta
NumberIsValid-8   0.00B          0.00B          ~     (all equal)

name             old allocs/op  new allocs/op  delta
NumberIsValid-8    0.00           0.00          ~     (all equal)

Change-Id: I4698c5db134998f83ff47fb3add6a04ba6ec3aa0

0 files changed, 0 insertions, 0 deletions