Improve symbol extraction performance

The performance of extraction of symbols from files has been improved. This is done by choosing a more efficient way to read the contents of files. Previously, file content was imploded to a string, by transforming an SplFileObject to an array, which slowed down during traversal of the iterator. This is switched out with a singular synchronous read on the file.

In some places, array_reduce is replaced by a foreach, to allow early breaks without additional function call overhead.

Thanks to @willemstuursma