`Vector.length` should have type `u32`

[DanielRX]

The following function in Vector:

/**
 * @returns The length of the vector.
 */
get length(): i32 {
  if (this._length < 0) {
    this._length = storage.get<i32>(this._lengthKey, 0);
  }
  return this._length;
}

should have a return type of u32 so that x.length can be compared against a u32 value (for(let i = 0; i <= x.length; i++))

There are two separate fixes for this, casting the type on return:

/**
 * @returns The length of the vector.
 */
get length(): u32 {
  if (this._length < 0) {
    this._length = storage.get<i32>(this._lengthKey, 0);
  }
  return <u32>this._length;
}

the other fix is to replace

this._length = -1

in the constructor with

this._length = storage.get<u32>(this._lengthKey, 0);

and changing the type of _length to u32

I can open a PR to fix either of these, but wanted to check which is more fitting with the style of the other code.