fix reading of xml escapes

[JanMarvin]

This is ycphs/openxlsx#394 (found it via https://asterhu.com/import-excel-formula-with-ampersand-to-r/)

Could you check if this solves your issue @aster-hu?

• add test
• compare speed with internal stringi solution
• use the faster one in other places as well? if not, why? to much hassle or generally safer?
• argument, is_xml to skip XML creation ?

[aster-hu]

It resolves my issue, thanks a lot!