Add a return Keyword

[smores56]

Zulip discussion

In order to facilitate early returns with the new try keyword, we want to add a more general return keyword that returns to the nearest function boundary. That means usages of return in an inline closure return to the closure's boundary, not the enclosing function's boundary.

Note to the implementer: We'll need to introduce a transformation into single static assignment (SSA) form for return usages either during or right before monomorphization. Feel free to ask for help in Zulip if you need context on that.