Fix path separators in proto-compiler

[greg-szabo]

There are some relative paths at the beginning of the main function in proto-compiler. To make the application cross-compatible on all platforms, the strings should be converted into PathBuf and instead of hard-coding / as the directory separator, the PathBuf::join method should be used to join the different directories together.

Low priority.

Requires #754 to be merged.
Enhances #723 .