Fix returning a constant

[mariari]

When writing the imperative example, I noticed a bug

(defcircuit imperative-example ((output int))
  (let ((accumulator 0))
    (loop for i from 0 to 9
          do (setf accumulator (+ i accumulator)))
    0))

If we return an explicit number

we get an error

The value
  0
is not of type
  SYMBOL
when binding PASS::SYMB
   [Condition of type TYPE-ERROR]

Restarts:
 0: [RETRY] Retry SLY mREPL evaluation request.
 1: [*ABORT] Return to SLY's top level.
 2: [ABORT] abort thread (#<THREAD "sly-channel-1-mrepl-remote-1" RUNNING {1001BB8073}>)

Backtrace:
 0: (PASS::RENAMING-SCHEME 0) [external]
 1: (PASS:RENAME-PRIMITIVE-CIRCUIT #<ALU.IR.PRIMITIVE-GLOBAL:PRIM-CIRCUIT IMPERATIVE-EXAMPLE -> 0 = ..)
 2: (ALU.PIPELINE.PIPELINE:TO-VAMPIR Circuit IMPERATIVE-EXAMPLE = ..)
 3: (ALU.PIPELINE.PIPELINE:PIPELINE Circuit IMPERATIVE-EXAMPLE = ..)
 4: (SB-INT:SIMPLE-EVAL-IN-LEXENV (VAMPIR IMPERATIVE-EXAMPLE) #<NULL-LEXENV>)
 5: (EVAL (VAMPIR IMPERATIVE-EXAMPLE))

; Returning var 0 of frame 1
#<ALU.IR.PRIMITIVE-GLOBAL:PRIM-CIRCUIT IMPERATIVE-EXAMPLE -> 0 =
(#<LET G1441 = #<#<REFERENCE +> 0 0>>
 #<LET G1442 = #<#<REFERENCE +> 1 #<REFERENCE G1441>>>
 #<LET G1443 = #<#<REFERENCE +> 2 #<REFERENCE G1442>>>
 #<LET G1444 = #<#<REFERENCE +> 3 #<REFERENCE G1443>>>
 #<LET G1445 = #<#<REFERENCE +> 4 #<REFERENCE G1444>>>
 #<LET G1446 = #<#<REFERENCE +> 5 #<REFERENCE G1445>>>
 #<LET G1447 = #<#<REFERENCE +> 6 #<REFERENCE G1446>>>
 #<LET G1448 = #<#<REFERENCE +> 7 #<REFERENCE G1447>>>
 #<LET G1449 = #<#<REFERENCE +> 8 #<REFERENCE G1448>>>
 #<LET G1450 = #<#<REFERENCE +> 9 #<REFERENCE G1449>>> #<STANDALONE-RET 0>) : (0)>

It seems the 0 doesn't get renamed for some reason, should be an easy fix