feat: Enhance Bound Enum with Utility Methods and Refactor Storable T…

…rait (#142)

### Description:

This PR introduces utility methods to the `Bound` enum, enhancing its
functionality and usability. These methods are used to refactor the
`Storable` trait, making it cleaner and more maintainable.

#### Use-Case:

I am developing a library aimed at simplifying the use of stable memory
structures. For example, consider a `TaskTimerEntry` struct defined as:

```rust
pub struct TaskTimerEntry<T> {
    pub time: NanoTimeStamp,
    pub task: T,
}
```

To make this struct bounded, I can now easily specify:

```rust
const BOUND: Bound = Bound::Bounded {
    is_fixed_size: false,
    max_size: 8 + T::BOUND.max_size(),
};
```

#### Key Changes:

Introduced `max_size` and  `is_fixed_size` methods to `Bound` enum.

#### Impact on `Storable` Trait:

The utility methods have been integrated into the `Storable` trait,
enhancing its readability and maintainability without altering its core
functionality.

#### Types of Changes:
- [x] New feature (non-breaking change which adds functionality)
- [ ] Bugfix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation Update (if none of the other choices apply)

src/storable.rs

@@ -70,6 +70,26 @@ pub enum Bound {
     },
 }
 
+impl Bound {
+    /// Returns the maximum size of the type if bounded, panics if unbounded.
+    pub const fn max_size(&self) -> u32 {
+        if let Bound::Bounded { max_size, .. } = self {
+            *max_size
+        } else {
+            panic!("Cannot get max size of unbounded type.");
+        }
+    }
+
+    /// Returns true if the type is fixed in size, false otherwise.
+    pub const fn is_fixed_size(&self) -> bool {
+        if let Bound::Bounded { is_fixed_size, .. } = self {
+            *is_fixed_size
+        } else {
+            false
+        }
+    }
+}
+
 /// Variable-size, but limited in capacity byte array.
 #[derive(Eq, Copy, Clone)]
 pub struct Blob<const N: usize> {