Add inital bytecode optimizations for #1163

This removes unnecessary movn, movf, lds, and a few other instructions.
Any instructions that has not side effects and writes to a slot that
isn't used can be removed. A number of other optimizations can follow
from this:

- Implement the def-aliasing-var optimization better
- This function can be iterated as a fix point until no more
  instructions are removed.
- If we implement slot renaming, then we no longer need to free slots
  and can simplify the initial code generation a lot.

test/suite0015.janet

@@ -4,7 +4,7 @@
 (start-suite 15)
 
 (assert (deep= (in (disasm (defn a [] (def x 10) x)) :symbolmap)
-               @[[0 3 0 'a] [1 3 1 'x]])
+               @[[0 2 0 'a] [0 2 1 'x]])
         "symbolslots when *debug* is true")
 
 (defn a [arg]
@@ -33,11 +33,11 @@
                                (def y 20)
                                (def z 30)
                                (+ x y z)))) :symbolmap)
-               @[[0 7 0 'arg]
-                 [0 7 1 'a]
-                 [1 7 2 'x]
-                 [2 7 3 'y]
-                 [3 7 4 'z]])
+               @[[0 6 0 'arg]
+                 [0 6 1 'a]
+                 [0 6 2 'x]
+                 [1 6 3 'y]
+                 [2 6 4 'z]])
         "arg & inner symbolslots")
 
 # buffer/push-at
@@ -45,4 +45,6 @@
 (assert (deep= @"abc456789" (buffer/push-at @"abc123" 3 "456789")) "buffer/push-at 2")
 (assert (deep= @"abc423" (buffer/push-at @"abc123" 3 "4")) "buffer/push-at 3")
 
+(assert (= 10 (do (var x 10) (def y x) (++ x) y)) "no invalid aliasing")
+
 (end-suite)