Deprecation list for `1.1.0`

[oleksiyskononenko]

• dt.isna(), dt.abs(), dt.exp(), dt.log(), dt.log10() are marked internally as deprecated for a long time, so they should be replaced with the corresponding dt.math.*() functions;
• dt.len() should be deprecated as it is replaced by dt.str.len();
• dt.frame_columns_virtual() functionality should be moved to dt.Frame class;
• .stype, .stypes, .ltype and .ltypes functionality should be accessible through .type/.types.