fix(kernel): add declarations for operator<<(std::ostream&, expr const&) and operator<<(std::ostream&, context const&) in the kernel

The actual implementation of these two operators is outside of the
kernel. They are implemented in the file 'library/printer.cpp'.
We declare them in the kernel to prevent the following problem.
Suppose there is a file 'foo.cpp' that does not include
'library/printer.h', but contains

    expr a;
    ...
    std::cout << a << "\n";
    ...

The compiler does not generate an error message. It silently uses the
operator bool() to coerce the expression into a Boolean. This produces
counter-intuitive behavior, and may confuse developers.