Context extraction

[nikku]

Which issue does this PR address?

This PR attempts to recognize the special context(entries) function, creating a context from a list of entries:

Closes #40

---

Some uses of context attached that are not recognized today, but would be on proper context extraction:

context([
  { key: "b+", value: { c-: 1} }
]).b+.c-

{
  entries: [
    {
      "key": "a + b",
      "value": 1
    },
    {
      "key": "b--",
      "value": 2
    }
  ],
  ctx: context(entries),
  value: ctx.a + b + ctx.b--
}

{
  dynamicEntry: {
    "key": "a + b",
    "value": {
      d-e: 1000
    }
  },
  entries: [
    dynamicEntry
  ],
  ctx: context(entries),
  value: ctx.a+b.d-e
}

{
  dynamicKey: "a + b",
  dynamicValue: {
    d-e: 1000
  },
  entries: [
    {
      "key": dynamicKey,
      "value": dynamicValue
    }
  ],
  ctx: context(entries),
  value: ctx.a + b.d-e
}