Optimize JSON serialization

[lahma]

• optimize ObjectInstance.GetOwnPropertyKeys for non-sorting case
• increase StringBuilderPool max capacity check
• smarter checks for integer numbers in JsonParser

Jint.Benchmark.JsonBenchmark

Diff	Method	FileName	Mean	Error	Allocated
Old	Parse	bestbuy_dataset.json	297.742 ms	5.7835 ms	115.01 MB
New			299.811 ms (+1%)	4.3964 ms	114.15 MB (-1%)
Old	Stringify	bestbuy_dataset.json	142.319 ms	2.2880 ms	104.63 MB
New			127.802 ms (-10%)	1.9511 ms	90.18 MB (-14%)
Old	Parse	twitter.json	3.023 ms	0.0057 ms	3.64 MB
New			2.995 ms (-1%)	0.0111 ms	3.61 MB (-1%)
Old	Stringify	twitter.json	2.926 ms	0.0241 ms	2.74 MB
New			2.434 ms (-17%)	0.0118 ms	1.4 MB (-49%)

/cc @tomatosalat0