[Util] Add Pooled::assert_not_none().

Converts an optional to a non-optional `Pooled`.  In a debug build,
asserts that a value is indeed present.

include/mutable/util/Pool.hpp

@@ -232,6 +232,11 @@ struct Pooled
 
     bool has_value() const requires can_be_none { return ref_ != nullptr; }
 
+    Pooled<T, Pool, false> assert_not_none() const requires can_be_none {
+        M_insist(has_value());
+        return { pool_, ref_ };
+    }
+
     /**
      * Returns the number of references to the pooled object or 0 if
      * this `Pooled` CanBeNone and does *not* hold a reference to an object.

unittest/util/PoolTest.cpp

@@ -151,13 +151,15 @@ TEST_CASE("PooledOptionalString Utilization", "[core][util][pool]")
         PooledOptionalString ps0{ pool("ps0") };
         REQUIRE(pool.size() == 1);
         REQUIRE(ps0.has_value());
+        CHECK(*ps0.assert_not_none() == "ps0"sv);
         REQUIRE(ps0.count() == 1);
 
         // copy constructing a non-empty PooledOptionalString should not affect the pool
         PooledOptionalString ps1{ ps0 };
         REQUIRE(pool.size() == 1);
         REQUIRE(ps0.has_value());
         REQUIRE(ps1.has_value());
+        CHECK(*ps1.assert_not_none() == "ps0"sv);
         REQUIRE(ps0 == ps1);
         REQUIRE(ps0.count() == 2);
         REQUIRE(ps1.count() == 2);