Rework schema and metadata

[Zruty0]

We wanted to change the following about the schema:

• Formally make it a collection of 'columns'.
• Turn metadata into an instance of IRow.
• (maybe) introduce 'global metadata' in addition to 'column metadata'.
  Below is the sketch of the new interface for schema:

    public interface ISchema : IReadOnlyList<ISchemaColumn>
    {
        /// <summary>
        /// Dataset-wide metadata.
        /// </summary>
        IRow Metadata { get; }
    }

    public interface ISchemaColumn
    {
        /// <summary>
        /// The name of a column. This string should always be non-empty.
        /// </summary>
        string Name { get; }

        /// <summary>
        /// The type of the column.
        /// </summary>
        ColumnType Type { get; }

        /// <summary>
        /// The metadata for a column, or <c>null</c> if this column has no metadata.
        /// </summary>
        IRow Metadata { get; }
    }

In addition, we might want to rework IRow to not include the notion of 'Row ID' and 'active columns'. Maybe this calls for another interface, like IMovingRow that would include these.

/cc @TomFinley

[TomFinley]

Thanks @Zruty0 . As some exploratory work which feels like some years ago, I introduced this file:

machinelearning/src/Microsoft.ML.Data/Data/IColumn.cs

Line 11 in 98ac03b

namespace Microsoft.ML.Runtime.Data

That might serve as a loose template for what we might want to do, and at least the work to do this should be aware of that.