Store Nix integers as longs

So on 64-bit systems, integers are now 64-bit.

Fixes #158.

src/libexpr/lexer.l

@@ -110,8 +110,10 @@ or          { return OR_KW; }
 \+\+        { return CONCAT; }
 
 {ID}        { yylval->id = strdup(yytext); return ID; }
-{INT}       { int n = atoi(yytext); /* !!! overflow */
-              yylval->n = n;
+{INT}       { errno = 0;
+              yylval->n = strtol(yytext, 0, 10);
+              if (errno != 0)
+                  throw ParseError(format("invalid integer `%1%'") % yytext);
               return INT;
             }