[ruby/json] Reduce comparisons when parsing numbers

Before this commit, we would try to scan for a float, then if that
failed, scan for an integer.  But floats and integers have many bytes in
common, so we would end up scanning the same bytes multiple times.

This patch combines integer and float scanning machines so that we only
have to scan bytes once.  If the machine finds "float parts", then it
executes the "isFloat" transition in the machine, which sets a boolean
letting us know that the parser found a float.

If we didn't find a float, but we did match, then we know it's an int.

ruby/json@0c0e0930cd

Author: tenderlove