define strict constructors policy for numbers
add UNumber.wrap() method for strings
implement mod() method (inexact, it's a remainder actually)
simplify UFloat implementation
implement glue between numbers and units (`UUnitNum` class)
units package uses `UNumber` for all operations
add binary and, binary or and remainder operations
sort out widgets and views into separate packages
add build target to start application
factor out keypad into separate layout