Add compressed ZIP copying

[klauspost]

Allow copying content from an existing ZIP file without decompressing+compressing.

New API:

// Copy will copy raw content from input file.
// Optionally a different name can be given to the new file.
func (w *Writer) Copy(name string, src *File) error

// OpenRaw returns a Reader that returns the *compressed* output of the file.
func (f *File) OpenRaw() (io.Reader, error)

// CreateHeaderRaw adds a file to the zip archive using the provided FileHeader
// for the file metadata. Writer takes ownership of fh and may mutate
// its fields. The caller must not modify fh after calling CreateHeaderRaw.
//
// This returns a Writer to which the compressed file contents should be written.
// The file's contents must be written to the io.Writer before the next
// call to Create, Copy, CreateHeader, CreateHeaderRaw or Close.
//
// Using this requires knowledge of populating the FileHeader correctly.
// Generally using the Copy() function is recommended.
func (w *Writer) CreateHeaderRaw(fh *FileHeader) (io.Writer, error)