Source file processing order not determined by command-line

[Jesper Öqvist]

Source files are not processed in the order they are listed on the command-line. The simple fix is to change the files set in Options to be a LinkedHashSet so that the iteration order is same as the initial insertion order.

The source file processing order is important when regression testing error messages.

[Jesper Öqvist]

Process source files in command-line order

fixes issue #57 (bitbucket)

→ <<cset d17f7989a298>>