added function test with gcd calculation

code/gcd.txt

@@ -0,0 +1,39 @@
+>main
+gto
+:gcd
+=v
+=u
+$u
+$v
+>loop
+gge
+$u
+=t
+$v
+=u
+$t
+=v
+:loop
+$v
+0
+>ret
+gle
+$v
+=t
+$u
+$v
+mod
+=v
+$t
+=u
+>loop
+gto
+:ret
+$u
+ret
+:main
+4
+16
+>gcd
+fun
+end