[Consistency Review] Map::get_or_default should use Default trait

[hackwaly]

Current behaviors

pub fn[T : Default] Option::unwrap_or_default(T?) -> T
pub fn[K : Hash + Eq, V] Map::get_or_default(Self[K, V], K, V) -> V

Expected behaviors

We should change Map::get_or_default and HashMap::get_or_default to pub fn[K : Hash + Eq, V: Default] Map::get_or_default(Self[K, V], K) -> V

Justification

[peter-jerry-ye]

The Map has a much broader use, so it is very unlikely that the default value designated by the default trait would fulfill the need.

[hackwaly]

Yes, the current version should be renamed as "Map::get_or" I think.