- use `@snippet` instead of `@code` in docs.
- remove some functions that were not used.

Modified performance test and solve a problem caused by an enum type.

For faster test

Major changes:

- modify the Base64 functions to compatible with `cvWriteRawData` and so
on.
- add a Base64 flag for FileStorage and outputs raw data in Base64
automatically.
- complete all  testing and documentation.