It's nice being able to do this now.
This starts to pave the path toward ultimately sharing code with Ulambda.
* build-aux/bootstrap/rebirth.scm: Extract steps 0--2 into separate source
files.
* build-aux/bootstrap/rebirth/es.scm: New file containing step 2.
* build-aux/bootstrap/rebirth/macro.scm: New file containing step 1.
* build-aux/bootstrap/rebirth/relibprebirth.scm: New file contaiing step 0.