Fix a crash on Wine due to an unimplemented MSVCP100 function.

When attempting to run a mod in SADX on Wine, the game crashes due to
a missing MSVCP100 function:

wine: Call from 0x7ed7cef6 to unimplemented function msvcp100.dll.?swap@?$basic_istream@DU?$char_traits@D@std@@@std@@QAEXAAV12@@Z, aborting
wine: Unimplemented function msvcp100.dll.?swap@?$basic_istream@DU?$char_traits@D@std@@@std@@QAEXAAV12@@Z called at address 0x7ed7cef6 (thread 0009), starting debugger...
Unhandled exception: unimplemented function msvcp100.dll.?swap@?$basic_istream@DU?$char_traits@D@std@@@std@@QAEXAAV12@@Z called in 32-bit code (0x7ed7cef6).

This can be worked around by using the original msvcp100.dll, but an
easier fix is to remove ifstream assignment. The ifstreams are now
constructed using an initializer, which not only fixes the mod loader
on wine, it's also slightly more efficient.