charset: clean up some code about charset

[xiongjiwei]

What problem does this PR solve?

• use a map to store the charset info directly
• remove Desc struct, it is definitely same with Charset

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

[ti-chi-bot]

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• tangenta
• xhebox

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[tangenta]

There are a few exported functions/struct changes:

• Desc/GetCharsetDesc
• GetSupportedCharsets
• GetCharsetInfo

This requires the users to change the code when they upgrade the dependency. I think this is acceptable since the changes should be easy. Please also update the TiDB part.

[xhebox]

Do we also cleanup collation same way?

[xiongjiwei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: caee1c0

[xiongjiwei]

Do we also cleanup collation same way?

I think the whole file should be rewritten. 😅