Implement memory mapping (mmap) to reduce memory usage when opening large XLSX files.
### Description ReadAll is used only in a few places, but most dominantly it is used to slurp up all the .xlsx we are opening. It may be only a few MiBs, but mmap is an easy solution for this. ### Steps to reproduce the issue 1. open a big .xlsx 2. Check used memory and GC churn ### Describe the results you received At least the size of .xlsx is used, then later may be released. ### Describe the results you expected Not a lot of memory is used, GC is in peace ### Go version 1.24.1 ### Excelize version or commit ID 2.9.0 ### Environment ```shell Debian Linux testing ``` ### Validations - [x] Check that there isn't already an issue that reports the same bug to avoid creating a duplicate. - [x] The provided reproduction is a minimal reproducible example of the bug.