Introduce the dev_println and dev_eprintln macros.

[mikebenfield]

If the dev-print feature is enabled, these work like println and eprintln, respectively. This is to give users (in particular Leo) more control of snarkvm's output - currently some print statements are compiled in when debug-assertions is on, but we want to elide those print statements even with debug-assertions.

I also removed some superfluous #[must_use] attributes - this attribute does nothing when used on a function in a trait implementation.