Replace Vunit::roots with a single entry block

Summary: We don't need multiple roots. All code should be reachable from a
single entry block. The only complication was bindaddr. It's now roughly
analogous to the other service request instructions in that it lives in the
code block that references it and directly emits the service request itself to
frozen.

Reviewed By: @edwinsmith

Differential Revision: D1603594