Add DiceServ: dice-roll and math-expression service

A stateless service for tabletop games over IRC. ROLL/CALC evaluate a dice
+ math expression (ROLL rounds to a whole number, CALC keeps decimals);
EXROLL/EXCALC also show each die. Supports NdM / dM / d% dice, + - * / ^ %,
parentheses, implicit multiplication, unary minus, pi/e, a set of functions
(sqrt, floor, sin, min, …), and N~expr to repeat a roll.

Modernises the C++ original: it ships its own Mersenne-Twister RNG in C
and parses via shunting-yard with a variadic-argument hack; this is a
typed AST from a small recursive-descent parser plus the rand crate —
shorter, and the dice/sides/total caps make abuse (999999d6) a clean error
instead of a hang. Opt-in (add "diceserv" to [modules].services).

src/main.rs

@@ -21,6 +21,7 @@ use fedserv_memoserv::MemoServ;
 use fedserv_statserv::StatServ;
 use fedserv_hostserv::HostServ;
 use fedserv_operserv::OperServ;
+use fedserv_diceserv::DiceServ;
 use fedserv_example::ExampleServ;
 use fedserv_inspircd::InspIrcd;
 use fedserv_nickserv::NickServ;
@@ -89,6 +90,11 @@ async fn main() -> Result<()> {
             uid: format!("{}AAAAAH", cfg.server.sid),
         }));
     }
+    if enabled("diceserv") {
+        services.push(Box::new(DiceServ {
+            uid: format!("{}AAAAAI", cfg.server.sid),
+        }));
+    }
     if enabled("example") {
         services.push(Box::new(ExampleServ {
             uid: format!("{}AAAAAC", cfg.server.sid),