Macro assistance part 2 - syntax for $(eval $(call...))

Macro assistance part 2 - syntax for $(eval $(call...))

This can be used to call a macro defined with &:macro.  In combination
these provide a more reasonable macro facility than make's.

Specifically:

* Document the new facility.
* Motivate it in the README.
* Provide the new @nest kind for &${  }, which uses $ddbl as
  a counter to count { and }.
* Implement the {}-counting and nesting end in the main parsing
  loop.  (We must now search for things besides $esc.)
* In extract-doctest, provide a slightly adhoc special case for
  understanding ${eval ...} as an expansion text in the README.
  And do not attempt to test &${ } when already $-doubling.

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