Lambda and Closure ~

A short way to create a function has been added: {args -> expr}. See |lambda|.
This is useful for functions such as `filter()` and `map()`, which now also
accept a function argument. Example: >
:call filter(mylist, {idx, val -> val > 20})

A lambda can use variables defined in the scope where the lambda is defined.
This is usually called a |closure|.

User defined functions can also be a closure by adding the "closure" argument
|:func-closure|.