Implement syntax extensions for #file and #line

[catamorphism]

Implement a syntax extension for #file (expanding to the name of the file that the extension is used in) and #line (expanding to the line number that the extension is used on). This is needed for #2228 and #2232.

[bstrie]

Is this also needed for #554?

[catamorphism]

@Wensleydale -- I don't think it's needed, strictly? Log doesn't seem to print out the file/line number where the log expression was right now, though it would be nice if it did.

[brson]

@paulstansifer I think this is done?.

[paulstansifer]

Closed by 88f4d06.