DCOM-309: Design metadata API to map 0..N properties to 0..M values

Jira issue originally created by user @ocramius:

We need a design for an API that allows converting (bi-directional) an arbitrary number of properties to an arbitrary number of values.

A rudimentary approach could be as following:

```
interface ToValueConverterInterface
{
// must be inlinable?
public function **invoke(
PropertiesMapInterface $properties // these are the properties in an object - not sure if we should pass in the object
) : ValuesMapInterface; // may be an array of scalars - not sure
}

interface ToObjectConverterInterface
{
// must be inlinable?
public function **invoke(
ValuesMapInterface $values // may be an array of scalars - not sure
) : PropertiesMapInterface; // these are the properties in an object - not sure if we should pass in the object
}

interface PropertiesMappingInterface
{
public function buildToValuesConverter(MapperContext $mapperCtx) : ToValueConverterInterface;
public function buildToObjectConverter(MapperContext $) : ToObjectConverterInterface;
}
```

`buildToValuesConverter` and `buildToObjectConverter` must be idempotent, so that we can generate code for each direction of the conversion via [DCOM-307](http://www.doctrine-project.org/jira/browse/DCOM-307) (Code inliner)