Fix constructors for structs with enum fields.

Fixes #1170.

Previously we were failing to identify that structs with enum fields
were movable. This led to compilation failures in cases where there
were APIs referring to Vec<SomeSuchStruct>, since we chose not to
impl CxxVector<SomeSuchStruct> as thought it was not movable.

This commit makes two changes to fix this:
* It considers that enums have implicit constructors.
* Previously, <Api as HasDependencies>::deps() was failing to list
  field or base dependencies for non-POD structs. This meant that
  depth_first failed to ensure that field types were considered
  prior to a struct in analyzing such implicit constructors.
  Normally this doesn't matter because types must be declared
  first, but in the case of nested or incomplete types this could
  have led to a faulty analysis.

engine/src/conversion/analysis/deps.rs

@@ -37,13 +37,9 @@ impl HasDependencies for Api<FnPrePhase1> {
                 ..
             } => Box::new(old_tyname.iter().chain(deps.iter())),
             Api::Struct {
-                analysis:
-                    PodAnalysis {
-                        kind: TypeKind::Pod,
-                        bases,
-                        field_deps,
-                        ..
-                    },
+                analysis: PodAnalysis {
+                    bases, field_deps, ..
+                },
                 ..
             } => Box::new(field_deps.iter().chain(bases.iter())),
             Api::Function { analysis, .. } => Box::new(analysis.deps.iter()),