Compute the length of a source file, counting every non-space character outside line comments and block comments.

The source is a list of lines. A line comment starts with // and runs to the end of the line; a block comment starts with /* and ends at the first */ and may span several lines. Everything inside a comment is ignored, so comments do not nest, and you need not handle comment markers inside string literals. Each non-space character outside comments adds 1 to the length.