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/primitives/icon.rs

@@ -53,6 +53,7 @@ icons! {
     AppWindow => "app-window",
     Archive => "archive",
     ArrowDownLeft => "arrow-down-left",
+    ArrowDownUp => "arrow-down-up",
     ArrowDownToDot => "arrow-down-to-dot",
     ArrowDownToLine => "arrow-down-to-line",
     ArrowDownWideNarrow => "arrow-down-wide-narrow",
@@ -131,6 +132,7 @@ icons! {
     Crop => "crop",
     Crosshair => "crosshair",
     Database => "database",
+    Delete => "delete",
     Dices => "dices",
     DollarSign => "dollar-sign",
     Dot => "dot",