Add ffi jit example.

examples/jitfn/hello.nasm

@@ -0,0 +1,17 @@
+BITS 64
+
+;;;
+;;; Code
+;;;
+mov rax, 1          ; write(
+mov rdi, 1          ;   STDOUT_FILENO,
+lea rsi, [rel msg]  ;   msg,
+mov rdx, msglen     ;   sizeof(msg)
+syscall             ; );
+ret                 ; return;
+
+;;;
+;;; Constants
+;;;
+msg: db "Hello, world!", 10
+msglen: equ $ - msg

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!")