Finish replacement of Dict by IndexMap

Main changes are in the collect rule and in the tests.

I could have just replaced

pub struct Dict {
    vec: Vec<(ast::Var, ast::ConstantRef)>,
    pub map: std::collections::HashMap<ast::Var, ast::ConstantRef>,
}

by

pub struct Dict {
pub map: indexmap::IndexMap<ast::Var, ast::ConstantRef>
}

to avoid the change of all the calls to interpret::Dict::new().put(...) but I think it's cleaner this way.