Fix missing method warning on build
Old entry writers refer to instead of copying the contents to reduce memory

* We already write entries to a temp file before overwriting the original file. Thus while writing local files, the contents memory map is still valid and the old entry writers need only refer to it. This should reduce memory usage even further.

* Contents and entries are now only invalidated after overwriting the original file. This ensures the old entry writers don't end up referring to unmapped content. Also after most update errors, the contents and entries are unchanged.

* Old entry writers only refer to their central header iff they can skip writing their local file. This is because writing the local file modifies the central header.