Final interface of Schema

[Zruty0]

As we have agreed on the API reviews, we will make the following modifications to Schema:

• Make it an ICollection<Schema.Column>
• Expand Column class to contain 2 fields: IsHidden and Index.
• Remove all methods except those exposed by collections, this accessors and GetColumnOrNull.
• Make constructors internal. Create a SchemaBuilder (in a different namespace Microsoft.ML.Schema) that would be responsible to building the schema.
  • This is to make it more obvious that columns are NOT reused by different schemas.
• Move Schema.Metadata.Builder into a separate class MetadataBuilder in namespace Microsoft.ML.Schema.
• Finally, probably rename to DataSchema

[TomFinley]

Hi @Zruty0 thanks for writing this. Pursuant to API review notes, let us consider the following refinements:

• this Column structure should probably be a struct.
• the enumerator over the columns should itself be a struct enumerator.
• you mentioned ICollection, but this interface contains methods for mutating (e.g., Add), so I think you meant something else. This should probably be refined here, since your intention was that Schema be immutable.
• Removal of value-tuple as an exchange type on this object, since this does not play very nicely with F#.

[sfilipi]

The only part that remains in this, is cleaning up usages of TryGetColumnIndex; which is not user-facing anymore; and renaming Schema to DataSchema.

[sfilipi]

All the work for this is done.
The only thing left atm, the renaming, is covered in #2297.