refactor(codec): improve typescript defs

[callmehiphop]

Relates to #223

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

---

This PR includes breaking changes!

Because Spanner rows can contain duplicate keys, the default format is an array of objects containing key value pairs.

const row = [
  {
    name: 'myfield',
    value: 'myvalue'
  }
];

Row arrays come equipped with a toJSON() method that transforms said arrays into a plain old JavaScript object while removing any custom wrappers (e.g. Spanner.int becomes number). However this was not applied recursively. The breaking changes do the following:

1. Unwrap nested values.
2. Transform Structs into plain old objects.

To elaborate on the second point, Structs behave exactly like rows and they can contain duplicate keys, so we've traditionally returned Arrays. However now they will return pojo's when toJSON() is called.

// before
const row = {
  mystruct: [
    {
      name: 'key',
      value: Spanner.int(10)
    }
  ]
};

// after
const row = {
  mystruct: {
    key: 10
  }
};

Similarly to wrapped numbers, we should allow the raw struct to be returned by passing a wrapStructs option.

row.toJSON({wrapStructs: true});

[JustinBeckwith]

I really don't have a good handle on what this code is doing, so it would be great if @stephenplusplus could review this as well :)