This is a single function, func IsValid([]byte) error, exposing some private
functionality in the go standard library encoding/json package. It does a
scan over encoded JSON data to make sure it is valid without too many
allocations.
The file scanner.go is copied directly from the json package. valid.go adds
the method, and valid_test.go contains a simple test.
scanner.go retains the BSD license from Go. The rest is also BSD licensed.