Make answer types truly generic

[phansch]

The answer type is currently hardcoded to be f32:

questionnaire/src/main.rs

Lines 18 to 20 in d0fbfb1

	// FIXME: This only works when all answers of type `Select` have the concrete
	// type `f32`. Need to abstract over that as well.
	questions.iter().for_each(|q: &Question<f32>| { map.insert(&q.name, q.ask()); } );

This prevents usage of 'select' answer types with choices of type other than f32.