Add support for "value" for SpecialExpr types

In particular, this lets us call "value" on instances of subclasses of
String, Symbol, and Pseudocode.

Author: d-torrance

M2/Macaulay2/d/interp.dd

@@ -538,6 +538,7 @@ value(e:Expr):Expr := (
 	       || err.message == breakMessage then if err.value == dummyExpr then nullE else err.value 
 	       else r)
 	  else r)
+     is x:SpecialExpr do value(x.e)
      else WrongArg(1,"a string, a symbol, or pseudocode"));
 setupfun("value'",value);
 

M2/Macaulay2/tests/normal/strings.m2

@@ -89,3 +89,4 @@ assert Equation(format(ascii(0..31) | "\"\\"),
 T = new SelfInitializingType of String
 assert Equation(net T "foo", "foo")
 assert Equation(format T "foo", "\"foo\"")
+assert Equation(value T "5", 5)