Update yara-x to 1.9.0; use new ScanFile functionality

[egibs]

This PR updates yara-x to 1.9.0 and uses the newly-added ScanFile method as opposed to the original Scan method which only supported a byte slice.

This allows yara-x to handle all of the memory mapping/file reading and cleans up a decent bit of code.

We still need the file contents for checksum calculations and report generation, so I updated the original syscall.Mmap+bytes.Clone functionality with bytes.Buffer+io.CopyBuffer instead.