Allow sign characters in integer literals.
Allow single-line comment to end at EOF.
Add sugar for definition parameters.
Add 'string' type and 'print' builtin.

 * Rename 'Program' => 'Fragment'

 * Evaluate fragments incrementally

 * Persist definitions between fragments

 * Add notion of a prelude (currently empty)
Qualify modules.
Move compilation and REPL out of 'Main'.