Task: Refactor SetOnce<T> to .NETify

[NightOwl888]

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Task description

SetOnce<T> has Get() and Set() methods without any overloads. This class can be improved by changing Get() and Set() methods to a Value property to match Nullable<T> in .NET.

Get() and Set() should be deprecated and moved to the Support/ObsoleteAPI (for removal in the rc1). This can be done by making SetOnce<T> into a partial class, which allows us to simply delete the whole class to remove the obsolete members.

NOTE: In Java, there was no restriction on which types could be used in SetOnce<T>, but all custom types in Java are reference types. Adapting this class to support value types doesn't seem worth the trouble. Users that need to have this functionality with numeric types can either use reference types from J2N.Numerics or make their own SetOnceInt32 type.