Alternative Go JSON parser

Supported types to unmarshal into:

• int
• float64
• string
• bool
• RawMessage
• structs made of all of these
• slices made of all of these
• pointers to all of these
• map[string]{all of these}

Other basic types would be easy to add; I have not needed them yet. Note that the empty interface is not supported.

• this works on a string, not a []byte
• encoding/json has nice error messages
• encoding/json can decode to the empty interface
• encoding/json supports the Unmarshaler interface

For the use case I'm mostly interested in (parsing openRTB requests) I get twice the performance and half the allocs:

BenchmarkRTB-4       	   50000	     29093 ns/op	    2536 B/op	      39 allocs/op
BenchmarkRTBThem-4   	   20000	     63858 ns/op	    3888 B/op	      87 allocs/op

Similar for parsing large ElasticSearch bulk results:

BenchmarkESbulk-4    	    3000	    473237 ns/op	  177979 B/op	     830 allocs/op
BenchmarkESbulkThem-4	    2000	   1055068 ns/op	  200884 B/op	    2283 allocs/op

Works fine for us.