Custom values comparer

[VitaliyMF]

Current comparison logic handles nulls in the following way:

• if a=null then "a==null" return true
• if a not null then "a>null" returns true

In some cases SQL-like behavior is desired when any comparison with null returns false. For this purpose

• lets enhance API to add ability to specify custom comparer
• add "NullBehaviour" property to ValueComparer implementation. By default current logic is preserved ("MinValue") and lets add "Sql" option that returns false if any operand is null.

[VitaliyMF]

Results:

• ValueComparer now is a public class and can be created outside
• ValueComparer was enhanced with options: FormatProvider (used for Convert.ChangeType), SuppressErrors (allows to compare non-compatible types without exception), NullComparison (determines how to compare with nulls)
• compiled expression cache becomes instance-specific (was static and shared across all LambdaParser instances): now compiled expression depends on the ValueComparer and global cache is irrelevant.