Add TryFullName to FSharpEntity

`FSharpEntity.FullName` is the only source of scope information in FCS.

We use `FSharpEntity.FullName` extensively in VFPT. It occasionally throws exceptions which lead to bad performance in tight loops.  

Although `TryFullName` is a bit ad-hoc, it gives reasonable performance in tight loops. `FullName` should still be used in normal circumstances.