ref(unreal): Update unreal parsing

[jan-auer]

This PR refactors the Unreal API and modernizes it a little.

Most important changes

• AppleCrashReports are now a separate file type that is detected in the unreal code. Previously, they were classified as Minidump.
• Parsing invalid unreal crash report files no longer panics, by switching from bytes::Buf to scroll.

Other changes

• Unreal4File now owns a handle to its data. Since the offsets are already verified when parsing the entire crash, one doesn't need to hold on to the entire crash.
• zlib implementation is now switched to flate2, which is also used in symbolic_debuginfo and in symbolicator. One less dependency.
• Removed some shortcut-methods that are superfluous. E.g. instead of providing a function Unreal4Crash.process_minidump, one can get the minidump file and then process it using ProcessState.from_minidump_buffer. This creates cleaner separation of concerns.
• Removed obsolete errors (one was dead before, one became dead by switching to scroll).

When updating symbolic prior to the 6.0 release, this was missed. Since this is a breaking change, we will likely have to bump symbolic to 7.0 after this.

[jan-auer]

This is final now. Going to merge once this has been implemented (and thus tried out) in Sentry.