Misc · Numbers: a calculator and a unit converter

misc::Calculator works a typed sum in forms::evaluate's precedence,
from a pad or the keyboard; Enter is equals while the calculator
itself has focus, and Escape clears or passes on. misc::UnitConverter
reads an amount in another unit across seven kinds, with a swap.
Both show numbers through format::significant, which rounds through
Rust's exponent form.

src/misc/mod.rs

@@ -1,7 +1,11 @@
+mod calculator;
 mod clock;
 mod stopwatch;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
+mod units;
 
+pub use calculator::Calculator;
 pub use clock::{Clock, WorldClock};
 pub use stopwatch::Stopwatch;
+pub use units::UnitConverter;