Syntax: Incompatible change: &$( does not do daft { thing

Syntax: Incompatible change: &$( does not do daft { thing

It is silly for &$( to expand to ${.  That requires the input to
contain non-matched kinds of brackets.

The shell compatibility is not needed.  In a shell rune, you can write
&\$thing since the { } are not normally needed there at all.  If they
are then now &\${thing} is necessary.  It would be possible to steal
"&${" (or allow only "&$(") but that seems less good.

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