Perf - FlexObject::TryGetMember double dictionary access

[drub0y]

The implementation of FlexObject::TryGetMember uses a pattern of calling ContainsKey to determine if the underlying Dictionary<K,V> contains a key and then, if it does, uses the indexer to actually get the value. This is more costly than it needs to be as it has to hash the key twice and do two hash lookups into the dictionary:

1. just to see if it contains the key
2. to go get the value

A better pattern for this is to use the Dictionary<K,V>::TryGetValue method which will simultaneously look up the key and, if it exists, return the value via the out parameter while simultaneously returning a bool to indicate whether or not an entry with the specified key exists.

NOTE: this technically only impacts users that choose to use the dynamic programming model when working with FlexBox types.

[cleemullins]

FlexObject has been removed by #425