Remove line number nodes and move the line number into statements

For a large IR line number nodes take up a significant portion of the memory footprint. We should get rid of the LineNumberNode altogether and only put them in Statements. I have a patch out for this already.

This has the added benefit that we don't need to do a lot of the special cases "look for the last statement, but it mustn't be a LineNumberNode", "pair function declaration vars with a linenumbernode" etc. So this both simplifies logic and footprint.
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