fix: do not trust spoofable Symbol.toStringTag in type dispatch

Every mode (src/index.js, src/full.js, src/lite.js, src/json.js)
picked its clone strategy from Object.prototype.toString.call(x),
which reads a user-settable Symbol.toStringTag. A plain object
declaring a recognized tag (eg "Map") was routed into that type's
clone branch and crashed on methods it doesn't have; one with an
unrecognized tag fell through every branch and was returned by
reference instead of cloned.

Also check the object's real constructor, which Symbol.toStringTag
cannot affect, so a plain object (or one with no prototype) is still
deep-cloned as a plain object no matter what tag it claims.

Fixes #54

test/suites/index.js

@@ -11,6 +11,7 @@ export { default as Sets } from './set';
 export { default as TypedArrays } from './typedarray';
 export { default as Descriptors } from './descriptor';
 export { default as Symbols } from './symbol';
+export { default as ToStringTag } from './tostringtag';
 
 export { default as Dates } from './date';
 export { default as RegExps } from './regexp';