Policies operate on solid type instead of interfaces

[ENikS]

Every policy in Unity operates on instances of solid type NamedTypeBuildKey. This requires allocating new instances every time type or name is used.

Using solid type does not allow optimizations where caller might already have all required information and could be simply cast into required type/name combo. To resolve this issue NamedTypeBuildKey should be changed to interface INamedType. Instead of

NamedTypeBuildKey SomePolicy(NamedTypeBuildKey buildKey, IBuilderContext context);

it should became

INamedType SomePolicy(INamedType buildKey, IBuilderContext context);

IBuilderContext should also use INamedType instead of NamedTypeBuildKey

public interface IBuilderContext
{
    ...
    INamedType OriginalBuildKey { get; }
    INamedType BuildKey { get; set; }
    ...