util: add isPrimitive check
smalloc: fix assertion fails/segfault

* Numeric values passed to alloc were converted to int32, not uint32
  before the range check, which allows wrap around on ToUint32. This
  would cause massive malloc calls and v8 fatal errors.
* dispose would not check if value was an Object, causing segfault if a
  Primitive was passed.
* kMaxLength was not enumerable.
smalloc: remove double checks

Now that values are checks in JS, no need for them to be checked in C++.