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/forms/mod.rs

@@ -85,6 +85,7 @@ pub(crate) use mention::{
     Suggestions, active_trigger, handles_matching, one_word, replace_trigger,
 };
 pub use multi::MultiSelect;
+pub(crate) use number::parse as parse_number;
 pub use number::{NumberInput, ScrubInput};
 pub use options::Choice;
 pub(crate) use options::{