- Added unit test for `base64_estimate_{de,en}code_size()`
- Mark constant stuff as `const`
- Use `memcmp()` for comparing memory
- Do not use variable size arrays
- Various code style issue
- suppressed cppcheck warning for `unassignedVariable` for
the encode/decode arrays in stream encode/decode tests
as these arrays are assigned by the according `base64_[en|de]code()` function.