Fix local variable declarations in the REPL.

A statement like:

  for (i in 1..2)

When run in the REPL declares a local variable ("i"), but not inside
a function or method body. This hit a corner case in the compiler
where it didn't have the correct slot indexes set up.

That corner case is because sometimes when you compile a chunk, local
slot zero is pre-allocated -- either to refer to "this" or to hold the
closure for a function so that it doesn't get GCed while running. But
if you're compiling top-level code, that slot isn't allocated. But top
level code for the REPL *should* be, because that gets invoked like a
function.

To simplify things, *every* compiled chunk now pre-allocates slot zero.
That way, there are fewer cases to keep in mind.

Also fixed an issue where a GC during an import could collected the
imported module body's closure.

Fix #456.

test/language/variable/many_locals.wren

@@ -1,6 +1,6 @@
 {
-  var a0 = "value"
-  var a1 = a0
+  // Slot zero is always taken to hold the closure or receiver.
+  var a1 = "value"
   var a2 = a1
   var a3 = a2
   var a4 = a3

test/language/variable/many_nonsimultaneous_locals.wren

@@ -1,10 +1,10 @@
-// Can have more than 256 local variables in a local scope, as long as they
+// Can have more than 255 local variables in a local scope, as long as they
 // aren't all in scope at the same time.
 
 {
   {
-    var a0 = "value a"
-    var a1 = a0
+    // Slot zero is always taken to hold the closure or receiver.
+    var a1 = "value a"
     var a2 = a1
     var a3 = a2
     var a4 = a3
@@ -263,8 +263,8 @@
   }
 
   {
-    var b0 = "value b"
-    var b1 = b0
+    // Slot zero is always taken to hold the closure or receiver.
+    var b1 = "value b"
     var b2 = b1
     var b3 = b2
     var b4 = b3

test/language/variable/too_many_locals.wren

@@ -1,6 +1,6 @@
 {
-  var a0 = "value"
-  var a1 = a0
+  // Slot zero is always taken to hold the closure or receiver.
+  var a1 = "value"
   var a2 = a1
   var a3 = a2
   var a4 = a3

test/language/variable/too_many_locals_nested.wren

@@ -1,6 +1,6 @@
 {
-  var a0 = "value"
-  var a1 = a0
+  // Slot zero is always taken to hold the closure or receiver.
+  var a1 = "value"
   var a2 = a1
   var a3 = a2
   var a4 = a3