Add optional buffer argument to Cipher#final

[segiddins]

This adds an optional buffer argument to the Cipher#final method, matching the behavior of Cipher#update. When a buffer is provided, the output is written to it and the same buffer object is returned. Otherwise, a new string is created and returned.

The implementation follows the same pattern as update:

• Accepts an optional string buffer parameter
• Reuses the buffer if provided, creating a new string if not
• Automatically resizes the buffer as needed
• Forces ASCII-8BIT encoding for binary cipher output
• Raises FrozenError if the buffer is frozen

[rhenium]

Why is this needed or desirable?

[segiddins]

It allows using a cipher to only require a single string allocation, by reusing the buffer passed to update, as opposed to allocating a second string inside final only to then append it to the buffer caller-side

[rhenium]

Have you measured the performance impact?

#final only allocates a small, usually RString-embedded string. My gut feeling is that avoiding this allocation isn't worth adding extra complexity to OpenSSL::Cipher, which is already fairly complicated.