Sweep guide rails and two-rail sweep (#771) (#1806)

Thanks @jyviko.

crates/doc/src/format/pending/sweep-guide-rails.rs

@@ -0,0 +1,5 @@
+//! A `Sweep` and a `SurfaceSweep` can carry guide `rails` (one or two, each a `sketch` and its
+//! `curves` in order) and `rail_fit` (`scale`, the default, or `stretch`): the profile follows
+//! the rails instead of an orientation, twist and end scale (#771). A sweep without them saves as
+//! it always did, so older designs need no transformation; 0.13.x asks for an update instead of
+//! sweeping a railed profile without its rails.