Add benchmark for large payloads with string escaping

[peterebden]

Illustrates the issue in #535

Output on my machine:

goos: linux
goarch: amd64
cpu: AMD Ryzen 9 5900X 12-Core Processor            
Benchmark_EscapedStrings_GoJson-24          	      1	27586403115 ns/op	23464256 B/op	 100081 allocs/op
Benchmark_EscapedStrings_EncodingJson-24    	     13	 82155502 ns/op	47859819 B/op	 300046 allocs/op
PASS
ok  	command-line-arguments	28.829s

Note that the two are pretty competitive if the payload size is reduced to 1000 objects. At 1000000 (10x this PR) it may as well be hanging.