Use Js.Types.symbol for Core__Symbol.t

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### API changes
 
 - Map, Set, WeakMap, WeakSet: use the types defined in the Js namespace. https://github.com/rescript-association/rescript-core/pull/143
+- Symbol: use the types defined in the Js namespace. https://github.com/rescript-association/rescript-core/pull/145
 - `Array` mutable & immutable helper name changed to conform to JS' upcoming APIs [such as `toSorted`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/toSorted)
   - `sort` -> `toSorted`, `sortInPlace` -> `sort`
   - `reverse` -> `toReversed`, `reverseInPlace` -> `reverse`

src/Core__Symbol.res

@@ -1,4 +1,4 @@
-type t
+type t = Js.Types.symbol
 
 @val external make: string => t = "Symbol"
 @val external getFor: string => t = "Symbol.for"

src/Core__Type.mjs

@@ -4,6 +4,11 @@
 function classify(value) {
   var match = Object.prototype.toString.call(value);
   switch (match) {
+    case "[object BigInt]" :
+        return {
+                TAG: /* BigInt */6,
+                _0: value
+              };
     case "[object Boolean]" :
         return {
                 TAG: /* Bool */0,