Ensure converters and comparers are handled by type mapping

[AndriySvyryd]

So we can change code like this:

property.GetValueComparer() ?? property.FindTypeMapping()?.Comparer
property.GetValueConverter() ?? property.FindTypeMapping()?.Converter

to just

property.FindTypeMapping()?.Comparer
property.FindTypeMapping()?.Converter

[ajcvickers]

@AndriySvyryd Current design allows the value comparer to overridden without needing to replace the entire type mapping. This is useful, for example, to do case insensitive string comparisons.

[AndriySvyryd]

Ok, but would it be that different if the type mapping was replaced (though since type mapping is done when the model is finalized there's nothing to replace)?
By grouping the same responsibilities into one class we improve cohesion and reduce error potential on usage.

[AndriySvyryd]

@ajcvickers This was done for GetValueComparer, but not for GetValueConverter