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.lock

@@ -328,6 +328,7 @@ dependencies = [
  "fedserv-api",
  "fedserv-botserv",
  "fedserv-chanserv",
+ "fedserv-diceserv",
  "fedserv-example",
  "fedserv-hostserv",
  "fedserv-inspircd",
@@ -372,6 +373,14 @@ dependencies = [
  "fedserv-api",
 ]
 
+[[package]]
+name = "fedserv-diceserv"
+version = "0.0.1"
+dependencies = [
+ "fedserv-api",
+ "rand",
+]
+
 [[package]]
 name = "fedserv-example"
 version = "0.0.1"