Cache commonly-used JSRT property keys (JsPropertyIdRefs)

[jasongin]

From #233 (comment)

Consider caching commonly-used property keys, instead of allocating them whenever they are used.

• In napi_define_class(): "prototype", "constructor"
• In napi_define_properties(): "get", "set", "value", "configurable", "enumerable", "writable"
• In napi_get_array_length(): "length"

[mhdawson]

Closing since Chakra is no longer active. Please let us know if you think that was not the right thing to do.