The simple fib test works

Running bytecode computed in the logic on the unverified CakeML bytecode
interpreter.  It's a trivial program, but it goes as fast as you would
hope:

Wernicke:tests sowens$ time ../../unverified/bytecode/cakeml-byte -b
fib.bbyte
val + = <fn>val - = <fn>val * = <fn>val div = <fn>val mod = <fn>val < =
<fn>val > = <fn>val <= = <fn>val >= = <fn>val = = <fn>val := = <fn>val ~
= <fn>val ! = <fn>val ref = <fn>val fib = <fn>val fibs =
<constructor>val _ = <constructor>val f20 = 6765
real	0m0.005s
user	0m0.001s
sys	0m0.002s

Unfortunately, running the compiler in the logic is slow:

time (do_compile_binary "fib.ml") "fib.bbyte"
runtime: 4m03s,    gctime: 3m53s,     systime: 6.918s.

I can't see how a program can generate enough garbage in 10 seconds to
need 4 minutes to collect it.  Something funny must be going on.