Add ref variables

[reviewed by hilde]

This patch adds references via the 'refvar' keyword to the language (to be
replaced by 'ref' in the future). References can only be set once, after
that any reads or writes occur on the referenced variable.

var x = 1;
refvar y = x;
writeln(y); // prints 1
x += 1;
writeln(y); // prints 2
y += 1;
writeln(y); // prints 3
writeln(x); // prints 3

Read-only references can also be created using 'const refvar':

var x = 1;
const refvar y = x;
writeln(y); // prints 1
x += 1;
writeln(y); // prints 2
y += 1; // Error


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23772 3a8e244f-b0f2-452b-bcba-4c88e055c3ca