Feature/skiplist

[fengkx]

What have you changed? (mandatory)

Add skiplist.

• Summarize your pull request (mandatory)
  Add skiplist to collections.
• How does this pull request work: a brief introduction for the changed logic (optional)
  A skiplist with insert, search, and remove.

NOTE: Do not assume that reviewers can understand the original issue

What is the type of the pull request? (mandatory)

The currently defined types are listed below, please pick one or more for this pull request

• New features (adds functionality)

• Improvement (an improvement to existing features)

• Fixing (fixes an issue)

• Documentation (improvements or changes to documentation)

Does this pull request affect documentation (docs)? (mandatory)

• If the documentation has changed, please open a new issue and add the pull request link that resolves the new issue here.
  No

Reference to a related pull request or issue link (optional)

Benchmark results (optional)