Add ZipArchive::closeToString()

[tstarling]

• Add a $flags parameter to ZipArchive::openString(), by analogy with ZipArchive::open(). This allows the string to be opened read/write.
• Have the $data parameter to ZipArchive::openString() default to an empty string, for convenience of callers that want to create an empty archive. This works on all versions of libzip since the change in 1.6.0 only applied to files, it's opt-in for generic sources.
• Add ZipArchive::closeToString() which closes the archive and returns the resulting string. For consistency with openString(), return an empty string if the archive is empty.
• Wrap strings passed to libzip with zip_source_function_create() instead of using zip_source_buffer_create(). This allows us to make the string writable, and simplifies memory management.

[alecpl]

Imo, either closeString() or saveToString().

[tstarling]

@ndossche It would be good to have your review on this.

@remicollet I considered making an openString() PR for the PECL module, but it doesn't merge cleanly and I'm touching the same lines of code again here, so I decided to do this PR first.

@alecpl I think closeString() would be fine, i.e. referring to the data model, instead of referring to what it returns. The new method can only be called when the underlying data is a string. But I'll give it a day to see if there are any other comments about this.