lighten up the Font repr
add bookends for text-storage mutation

- (though it’s still unclear to me whether the consistency checks it
performs make a difference since the layout manager isn’t hooked up to
an NSTextView…)
use the `indent` distance for tabs

- if the current indent is zero, \t chars will scoot rightward to the
next 1-em increment
- if the indent is non-zero, the \t increments will be set to multiples
of the indent
Text.used no longer bombs when empty