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).

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["api", "inspircd", "chanserv", "nickserv", "example", "botserv", "memoserv", "statserv", "hostserv", "operserv"]
+members = ["api", "inspircd", "chanserv", "nickserv", "example", "botserv", "memoserv", "statserv", "hostserv", "operserv", "diceserv"]
 
 [package]
 name = "fedserv"
@@ -18,6 +18,7 @@ fedserv-memoserv = { path = "memoserv" }
 fedserv-statserv = { path = "statserv" }
 fedserv-hostserv = { path = "hostserv" }
 fedserv-operserv = { path = "operserv" }
+fedserv-diceserv = { path = "diceserv" }
 tokio = { version = "1", features = ["net", "io-util", "rt-multi-thread", "macros", "time", "sync", "process"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"