`debug()` statement: show filename and line number

[ogoffart]

Commit 9fbb40d introduced a debug statement, but it still full of FIXME

The function arguments need to be handled. The backend need to be chosen.

ideally, debug("hello", 3+3); should print

foobar.slint:42: hello 6

But the backend should be selected. There could be a runtime function that actually do the print, and the generated code could call it.