Reduce visibility to private for some functions

[phsym]

Many functions today are public and visible to crate's users. Their usage is internal to the crate and they should be private instead.

As an example, this has already been reported in #57.

Let's warn users by deprecating them in a first time, and give them time to bring feedback if they eventually use some of those functions

This will have the advantages of simplifying the API and its documentation, and removing those internal functions from public API will make it easier to introduce bigger changes without breaking the public API.
After making them private, they can still be brought back to public later if needed.

List of deprecated function:

• Cell::print
• Cell::print_term
• Cell::get_height
• Cell::get_width
• LineSeparator::print (Already deprecrated. Now changed to private)
• TableFormat::print_line_separator
• TableFormat::print_column_separator
• TableSlice::get_column_num
• Table::get_column_num
• Row::column_count
• Row::get_height
• Row::get_column_width
• Row::print
• Row::print_term

If you already use some of the deprecated functions, and want them to stay public, let me know in comments