[elixir-core:11613] add Keyword.take!/2, Map.take!/2

[DaTrader]

Many times I need to merge a subset of a keyword list or a map into another 
keyword list or map while requiring it to fail if not all keys are present.

For this purpose I have small utility modules called KwUtils and MapUtils 
where I have functions like the one below:

```
  @doc """
  Takes specified key-value pairs from the keyword list but raises if any 
not found.
  """
  @spec take!( keyword(), [ atom()]) :: keyword()
  def take!( kw, keys) do
    taken = Keyword.take( kw, keys)

    if length( taken) != length( keys) do
      raise "Expected all keys in #{ inspect( keys)} to be present in #{ 
inspect( kw)}"
    end

    taken
  end
```

It'd be nice if a corresponding function was part of the `Keyword` and 
`Map` modules each.

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