feat: add ToMap for stream

[ppd324]

add ToMap for stream

[cannian1]

@ppd324 main branch was frozen, merge into rc branch.

[ppd324]

Is it necessary to add a merge function to the ToMap method to merge key conflicts?
like this:

func ToMap[T any, K comparable, V any](s Stream[T], mapper func(item T) (K, V), mergeFunc func(existing, new V) V) map[K]V {
	m := make(map[K]V, len(s.source))
	for _, v := range s.source {
		key, value := mapper(v)
		if existing, ok := m[key]; ok {
			if mergeFunc != nil {
				m[key] = mergeFunc(existing, value)
				continue
			}
		}
		m[key] = value
	}
	return m
} 

[cannian1]

func ToMap[T any, K comparable, V any](s Stream[T], mapper func(item T) (K, V), mergeFunc func(existing, new V) V) map[K]V {
	m := make(map[K]V, len(s.source))
	for _, v := range s.source {
		key, value := mapper(v)
		if existing, ok := m[key]; ok {
			if mergeFunc != nil {
				m[key] = mergeFunc(existing, value)
				continue
			}
		}
		m[key] = value
	}
	return m
} 

I think it's better to split this into two functions

Simpler API for common cases

func ToMap[T any, K comparable, V any](s Stream[T], mapper func(item T) (K, V)) map[K]V {
	return ToMapWithMerge(s, mapper, nil)
}

func ToMapWithMerge[T any, K comparable, V any](s Stream[T], mapper func(item T) (K, V), mergeFunc func(existing, new V) V) map[K]V