Add ffi jit example.

examples/jitfn/jitfn.janet

@@ -0,0 +1,12 @@
+###
+### Relies on NASM being installed to assemble code.
+### Only works on x86-64 Linux.
+###
+### Before running, compile hello.nasm to hello.bin with
+### $ nasm hello.nasm -o hello.bin
+
+(def bin (slurp "hello.bin"))
+(def f (ffi/jitfn bin))
+(def signature (ffi/signature :default :void))
+(ffi/call f signature)
+(print "called a jitted function with FFI!")