mapx: TreeMap 添加 Keys 和 Values 方法

[flycash]

将原本 Remove 方法改成了 Delete 方法，这算是一个不兼容的修改。不过之前 review 代码的时候没有注意，也没得办法

[flycash]

@juniaoshaonian

[codecov]

Codecov Report

Merging #181 (fb3444f) into dev (63b5735) will decrease coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #181      +/-   ##
==========================================
- Coverage   95.89%   95.58%   -0.31%     
==========================================
  Files          43       44       +1     
  Lines        2362     2402      +40     
==========================================
+ Hits         2265     2296      +31     
- Misses         77       83       +6     
- Partials       20       23       +3     

Impacted Files	Coverage Δ
mapx/hashmap.go	100.00% <ø> (ø)
internal/tree/red_black_tree.go	95.71% <100.00%> (+0.04%)	⬆️
mapx/treemap.go	100.00% <100.00%> (ø)
set/treeset.go	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[flycash]

组合形态会导致 RBTree 被暴露出去，这是我觉得不能接受的