Improve validation of impure expressions

Give a name to the special reference value for '$$'

Use madeline instead of a custom line editor

README: Fix Hare installation link

Update per for-each changes

Fix non-'$$' references

If the first character wasn't '$', it used to be silently discarded, causing
"$1" and similar to be a syntax error.

Make $$ return value of the last expression

Update for current Hare

Signed-off-by: Carlos Une <une@fastmail.fm>

rational: Use math::gcd

Update per stdlib changes

ui: Correctly display negative whole numbers

ui/print_rational: correctly handle whole integers

Previously 4 was displayed as -(4 + 0/1)

Update for memio

Simplify print_rational

README.md: add new option syntax

Add an option to disable preview

Refactor options

Stop showing empty expressions as errors

Implement wordwise motion

Implement factorials