Implement assumptions defined in the babel/rfcs#5 RFC

- `mutableTemplateObject` and `ignoreToPrimitiveHint` (#12408)
- `setClassMethods` (#12407)
- `setComputedProperties` (#12490)
- `ignoreFunctionLength` (#12491)
- `noDocumentAll` (#12481)
- `iterableIsArray` and `arrayLikeIsIterable` (#12489)
- `pureGetters` (#12504)
- `skipForOfIteratorClosing` (#12496)
- `objectRestNoSymbols`, `setSpreadProperties` and `pureGetters` (#12505)
- `noNewArrows` (#12613, #12793)
- `setPublicClassFields` and `privateFieldsAsProperties` (#12497)
- `constantReexports` and `enumerableModuleMeta` (#12618)
- `constantSuper`, `superIsCallableConstructor` and `noClassCalls` (#12726)

Co-authored-by: Justin Ridgewell <justin@ridgewell.name>
Co-authored-by: Huáng Jùnliàng <JLHwung@users.noreply.github.com>

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": ["external-helpers", "transform-spread"],
+  "assumptions": {
+    "iterableIsArray": true
+  }
+}