Reuse symbol collection namespace during typechecking

[jjcnn]

The symbol collection phase traverses the structure of the current package and keeps track of all name bindings using the namespace module. However, ths resulting structure is then discarded, and the typechecker rebuilds the structure:

    let collection_namespace = Namespace::new(handler, engines, initial_namespace.clone(), true)   // <-- USES CLONE OF initial_namespace
        .map_err(|error| TypeCheckFailed {
            root_module: None,
            namespace: initial_namespace.clone(),
            error,
        })?;
    // Collect the program symbols.

    let mut collection_ctx =
        ty::TyProgram::collect(handler, engines, parse_program, collection_namespace).map_err(
            |error| TypeCheckFailed {
                root_module: None,
                namespace: initial_namespace.clone(),
                error,
            },
        )?;

    let typecheck_namespace =
        Namespace::new(handler, engines, initial_namespace, true).map_err(|error| {   // <-- USES initial_namespace
            TypeCheckFailed {
                root_module: None,
                namespace: collection_ctx.namespace().root_ref().clone(),
                error,
            }
        })?;

Since initial_namespace contains all bindings in the dependency graph (minus the bindings in the current package), cloning the namespace is an expensive operation.

#6838 would make the call to initial_namespace.clone() significantly less expensive, but ideally we would like to reuse the namespace from the symbol collection phase when performing typechecking.