fix GC issue by globally rooting values

[ericphanson]

in #18 I did not use UnsafeArrays correctly: I assumed the gcref field would be enough to root the arrays to Julia's GC would not free the memory, making the pointer inside the UnsafeArray invalid. However it is not; Julia can figure out we never use gcref and free the memory, as the test/test_gc.jl file shows; it fails on main.

This fixes it, but at a big performance cost with

forward pass: 1.457254 seconds (5.59 k allocations: 11.969 MiB)
bumper_yolomod(b, batch_aa; detectThresh = 0.5, overlapThresh = 0.8): 1.500037 seconds (19.29 k allocations: 12.636 MiB)

compared to ~1s without allocarrays on r3tex/ObjectDetector.jl#101

[ericphanson]

this is a worse solution than just doing less unsafe stuff: closing in favor of #20