Fix issues around Xcode 16, and rename to parse(...) from init(...)

[rinsuki]

We shouldn't depends to pointer is still valid after withUnsafe~'s callback is ended (and Xcode 16's swiftc finally breaks it).

but convenience init will not allow us to call self.init from withUnsafe~'s callback, so there are two options:

• Allocate temporary memory, copy input to it, feed it to libxml2, then free it
  • This is what Fix for Xcode16: safe way to create UnsafeBufferPointer #131 does
  • Pros: no need to change API
  • Cons: If someone parsing big XML/HTML document in memory-limited environment like an iOS App Extension, they might be want to avoid additional copy
• Rename to parse(...) from init(...) and call libxml2 inside of withUnsafe~ callback
  • This is what this pull-request does
  • Pros: No additional copy/allocate in Fuzi part
  • Cons: breaking changes to our API