Rename Features::active_features.

The word "active" is currently used in two different and confusing ways:
- `ACTIVE_FEATURES` actually means "available unstable features"
- `Features::active_features` actually means "features declared in the
  crate's code", which can include feature within `ACTIVE_FEATURES` but
  also others.

(This is also distinct from "enabled" features which includes declared
features but also some edition-specific features automatically enabled
depending on the edition in use.)

This commit changes the `Features::active_features` to
`Features::declared_features` which actually matches its meaning.
Likewise, `Features::active` becomes `Features::declared`.

Author: nnethercote

clippy_lints/src/manual_float_methods.rs

@@ -85,7 +85,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualFloatMethods {
         if !in_external_macro(cx.sess(), expr.span)
             && (
                 matches!(cx.tcx.constness(cx.tcx.hir().enclosing_body_owner(expr.hir_id)), Constness::NotConst)
-                    || cx.tcx.features().active(sym!(const_float_classify))
+                    || cx.tcx.features().declared(sym!(const_float_classify))
             ) && let ExprKind::Binary(kind, lhs, rhs) = expr.kind
             && let ExprKind::Binary(lhs_kind, lhs_lhs, lhs_rhs) = lhs.kind
             && let ExprKind::Binary(rhs_kind, rhs_lhs, rhs_rhs) = rhs.kind