Encode section lengths with unsigned LEB128

asm.js

@@ -1327,7 +1327,9 @@ export class Assembler {
         ];
         const sections = template.map(([ code, generator ]) => {
             const body = generator();
-            return body == null ? [] : [ code, body.length, body ];
+            if (body == null)
+                return [];
+            return [ code, this.uleb128(body.length), body ];
         });
 
 		return new Uint8Array([ HEADER, sections ].flat(Infinity));