Extended debug info

[vtereshkov]

Currently the debug info consists of only the module name, function name and line number. This is sufficient for printing meaningful runtime error messages, but not for step-by-step debugging. In particular, the 'Watch' command cannot be implemented, as the local variables' names and types are lost after the program source has been compiled into bytecode.

Even though it's pretty easy to implement line-based debug hooks needed by the 'Step into' and 'Step over' debugger commands, these commands will be almost useless without the 'Watch' command.

It is also unclear how to provide the extended debug info via the C API.