refactor: flattenToken use WeakMap

[zombieJ]

For large app, flatten token for each component will repeat called multiple times. Use WeakMap to cache the result:

[github-actions[bot]]

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[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (eeb7455) 94.51% compared to head (3015ed5) 94.53%.

:exclamation: Current head 3015ed5 differs from pull request most recent head ab8fc3d. Consider uploading reports for the commit ab8fc3d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #143 +/- ## ========================================== + Coverage 94.51% 94.53% +0.02% ========================================== Files 25 25 Lines 2170 2179 +9 Branches 342 344 +2 ========================================== + Hits 2051 2060 +9 Misses 119 119 ``` | [Files Changed](https://app.codecov.io/gh/ant-design/cssinjs/pull/143?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ant-design) | Coverage Δ | | |---|---|---| | [src/util.ts](https://app.codecov.io/gh/ant-design/cssinjs/pull/143?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ant-design#diff-c3JjL3V0aWwudHM=) | `45.00% <100.00%> (+4.45%)` | :arrow_up: |

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