Syntax: Use &\ for escaping; abolish \&

"\&" causes lots of trouble, including difficulty specifying certain
resulting strings.

Previously I proposed a scheme using &$@ but this is only useful to
provide a way to write &s.

The $-doubling regime which will come later wants a way to produce a
single literal $, so introduce that now using the same syntax.
And &\ feels quite natural to me.

Incompatible change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>