reduce excess print statements

[droundy]

I've been meaning for some time now to reduce the amount of visual noise when running fac --continual. The simplest way to do this is to convert println! macros to vprintln!... or even vvprintln! if it seems particularly unimportant. The vprintln! family are defined in src/build/mod.rs as I recall, and just restrict the printing to the case where fac was called with the --verbose flag.