stream_variable inline declaration causes link error

[daniel-larraz]

The stream_variable function is declared inline only in the header file:

libpoly/include/polyxx/variable.h

Lines 50 to 51 in fc54e31

	inline detail::variable_printer stream_variable(const Context& c,
	const Variable& v);

Because the function is marked inline without an in-header definition, this can sometimes cause undefined reference errors when linking.

Two potential fixes are:

1. Remove the inline qualifier from the declaration.
2. Keep inline but move the definition into the header, for example:

    inline detail::variable_printer stream_variable(const Context& c,
                                                    const Variable& v)
    {
        return detail::variable_printer{c.get_variable_db(), v.get_internal()};
    }

Would it be possible to incorporate one of these fixes?

[daniel-larraz]

Fixed by #99