feat(shape, dynamodb): Pick/Extend mechanics for Records to support DynamoDB Indexes

[sam-goodwin]

Closes #104

Two new mechanics for Records:

• Extends a Record to create a child record class - sub-typing! any suggestions for better name?

  class A extends Record({
  a: string
  }) {}
  // extend A to create B
  class B extends A.Extends({
  b: string
  }) {}
  // equiv. to:
  type B = A & {
  b: string
  }

• Pick members from the Record (useful for DDB projections)

  class BOnly extends B.Pick(['b']) {}
  // equiv to.
  type BOnly = Pick<B, 'b'>

DynamoDB Indexes and Projections

This is then used to support Projections in DynamoDB Indexes.

class Counter extends Record({
  key: string,
  count: integer,
  data: binary, // large amount of binary data
}) {}

const hashTable = new DynamoDB.Table(stack, 'Table', {
  data: Counter,
  key: {
    partition: 'key'
  }
});

const countIndex = hashTable.globalIndex({
  indexName: 'index',
  key: {
    partition: 'count',
    sort: 'key'
  }
});

Projections:

class CounterProjection extends Counter.Pick(['key', 'count']) {}

const countIndex = hashTable.projectTo(CounterProjection).globalIndex({
  indexName: 'index',
  key: {
    partition: 'count',
    sort: 'key'
  }
});