Faster side table

Instead of using hasOwnProperty we store an entry array and compare the key.

This makes `get` about 3x faster in V8, 2x faster in JSC, 8x faster in SpiderMonkey, 2x in Chakra.
This makes `set` about 21x faster in V8, 7x faster in JSC, 22x faster in SpiderMonkey, 3x in Chakra.
http://jsperf.com/hasown/2
http://jsperf.com/hasown/3

Note that these results are micro benchmarks so they should be taken with a grain of salt.