render: add static_ combinator

static_ is the equivalent of append except that it will append an
element only when its parent is created, and never after. Any selections
chained below the result will still be executed on every update.

Currently this is called static_ because of a bug in the Elm compiler.
Once that's resolved (already fixed in dev), it will be renamed
'static'.
render: drop root selection argument of render

Having separate selection arguments for the root and the body is no
longer necessary now that static_ exists.
render: Add nest operator

The nest operator is useful for composing views at the top level. It
will apply the first selection to the context to create a new context.
The second selection is then applied to the new context, with the new
context being the result of the entire expression.

It's not strictly necesary to implement nest nateively. It's actually
equivalent to the following:

  nest a b = a <.> (sequence update b)
render: document context updates