Ensure correct variant count in Runtime[Hold/Freeze]Reason (#1900)

closes https://github.com/paritytech/polkadot-sdk/issues/1882

## Breaking Changes

This PR introduces a new item to `pallet_balances::Config`:

```diff
trait Config {
++    type RuntimeFreezeReasons;
}
```

This value is only used to check it against `type MaxFreeze`. A similar
check has been added for `MaxHolds` against `RuntimeHoldReasons`, which
is already given to `pallet_balances`.

In all contexts, you should pass the real `RuntimeFreezeReasons`
generated by `construct_runtime` to `type RuntimeFreezeReasons`. Passing
`()` would also work, but it would imply that the runtime uses no
freezes at all.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/support/src/traits.rs

@@ -61,7 +61,7 @@ pub use misc::{
 	DefensiveTruncateFrom, EnsureInherentsAreFirst, EqualPrivilegeOnly, EstimateCallFee,
 	ExecuteBlock, ExtrinsicCall, Get, GetBacking, GetDefault, HandleLifetime, IsSubType, IsType,
 	Len, OffchainWorker, OnKilledAccount, OnNewAccount, PrivilegeCmp, SameOrOther, Time,
-	TryCollect, TryDrop, TypedGet, UnixTime, WrapperKeepOpaque, WrapperOpaque,
+	TryCollect, TryDrop, TypedGet, UnixTime, VariantCount, WrapperKeepOpaque, WrapperOpaque,
 };
 #[allow(deprecated)]
 pub use misc::{PreimageProvider, PreimageRecipient};

substrate/frame/support/src/traits/misc.rs

@@ -36,6 +36,18 @@ pub const DEFENSIVE_OP_PUBLIC_ERROR: &str = "a defensive failure has been trigge
 #[doc(hidden)]
 pub const DEFENSIVE_OP_INTERNAL_ERROR: &str = "Defensive failure has been triggered!";
 
+/// Trait to get the number of variants in any enum.
+///
+/// NOTE: can be removed once <https://doc.rust-lang.org/std/mem/fn.variant_count.html> is stable.
+pub trait VariantCount {
+	/// Get the number of variants.
+	const VARIANT_COUNT: u32;
+}
+
+impl VariantCount for () {
+	const VARIANT_COUNT: u32 = 0;
+}
+
 /// Generic function to mark an execution path as ONLY defensive.
 ///
 /// Similar to mark a match arm or `if/else` branch as `unreachable!`.