Refactoring: made TypeSpecificationEmit more clear by renaming IncludeField to GenerateColumnHandle and GetColumnHandle to LookupColumnHandle.
Tweaked TypeSpecification.SetColumnIndex to allow it to be called and not react to a field not being present (to be used for implicit fields).
Extended the code host HostManager to assign runtime column handles for fields mapping against implicit columns.
Changed the weaver so that it generates column handles for fields that map against implicit columns if such fields are found.
Changed the TypeSpecificationEmit.GenerateColumnHandle method to accept name of the field instead of field reference, since we don't always have one (for implicit fields).
Removed constraints in weaver DynamicTypesHelper class relating to multiple declarations of entity-related fields in hierarchies; now that these are treated similar to synonyms, we can allow several.