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.

tasks/ch31-43.md

@@ -315,8 +315,8 @@
 
 ## T42b Misc · Numbers
 
-- [ ] Calculator
-- [ ] UnitConverter
+- [x] Calculator → `misc::Calculator`: digits, the four operations in precedence through `forms::evaluate`, sign, delete and clear, twelve significant digits; keys type too, and Enter gives the result while the calculator itself has focus
+- [x] UnitConverter → `misc::UnitConverter`: length, area, volume, mass, temperature, speed and data, each on a starting pair, with a swap; six significant digits
 
 ## T42c Misc · Codes and checks