Simplify how token strings are converted

No longer need to-string when using concat. However, it is still needed if the token string consists of a single token.

src/style-spec/function/convert.js

@@ -226,7 +226,7 @@ export function convertTokenString(s: string) {
         const literal = s.slice(pos, re.lastIndex - match[0].length);
         pos = re.lastIndex;
         if (literal.length > 0) result.push(literal);
-        result.push(['to-string', ['get', match[1]]]);
+        result.push(['get', match[1]]);
     }
 
     if (result.length === 1) {
@@ -236,7 +236,7 @@ export function convertTokenString(s: string) {
     if (pos < s.length) {
         result.push(s.slice(pos));
     } else if (result.length === 2) {
-        return result[1];
+        return ['to-string', result[1]];
     }
 
     return result;

test/integration/expression-tests/legacy/interval/tokens-zoom/test.json

@@ -1,8 +1,8 @@
 {
-  "expression": {"type": "interval", "stops": [[0, "0 {a}"], [1, "1 {b}"]]},
+  "expression": {"type": "interval", "stops": [[0, "0 {a}"], [1, "{b}"]]},
   "inputs": [
-    [{"zoom": 0}, {"properties": {"a": "a", "b": "b"}}],
-    [{"zoom": 1}, {"properties": {"a": "a", "b": "b"}}],
+    [{"zoom": 0}, {"properties": {"a": "a", "b": 2}}],
+    [{"zoom": 1}, {"properties": {"a": "a", "b": 2}}],
     [{"zoom": 0}, {"properties": {}}]
   ],
   "propertySpec": {
@@ -18,13 +18,13 @@
       "isZoomConstant": false,
       "type": "string"
     },
-    "outputs": ["0 a", "1 b", "0 "],
+    "outputs": ["0 a", "2", "0 "],
     "serialized": [
       "step",
       ["zoom"],
-      ["concat", "0 ", ["to-string", ["get", "a"]]],
+      ["concat", "0 ", ["get", "a"]],
       1,
-      ["concat", "1 ", ["to-string", ["get", "b"]]]
+      ["to-string", ["get", "b"]]
     ]
   }
 }