Read only first two bytes to check signature

[aidenfoxivey]

When using stackprof on a large trace (approximately 3.5 GiB), I noted that stackprof fails.

/opt/homebrew/lib/ruby/gems/3.4.0/gems/stackprof-0.2.27/lib/stackprof/report.rb:13:in 'IO.binread': Invalid argument @ io_fread - /Users/aidenfoxivey/src/ruby-bench/benchmarks/lobsters/zjit_exit_locations.dump (Errno::EINVAL)
        from /opt/homebrew/lib/ruby/gems/3.4.0/gems/stackprof-0.2.27/lib/stackprof/report.rb:13:in 'StackProf::Report.from_file'
        from ./bin/stackprof:78:in '<main>'

This is because IO.binread can only read a max size of 2 GiB (the number of bytes there being equivalent to the max size of a signed 32-bit integer).

Since checking the first two bytes should not require reading the whole file, this patch only reads the first two, rewinds, and then (if matching the MARSHAL_SIGNATURE) uses Marshal.load, which does allow reads over 2 GiB.