Add new() function to StorageKey

[bitzoic]

Description

When creating complex storage types, storing of a StorageKey is sometimes needed. In these cases a new() function is appropriate.

For example, when we have the following storage block:

storage {
    name: StorageMap<u64, StorageKey<StorageString>> = StorageMap {},
}

In order to store a StorageString in a StorageMap, the StorageKey<StorageString> annotation is needed.

From there, we can store a String by creating a StorageKey, storing a String at said key, and then storing the StorageKey in the StorageMap:

let string_key: StorageKey<StorageString> = StorageKey {
    slot: sha256(0),
    offset: 0,
    field_id: sha256(1),
};
string_key.write_slice(String::from_ascii_str("my_name"));
storage.name.insert(0, string_key);

The introduction of new() would change this to the following:

let string_key: StorageKey<StorageString> = StorageKey::new(
     sha256(0), 
     0, 
     sha256(1)
);
string_key.write_slice(String::from_ascii_str("my_name"));
storage.name.insert(0, string_key);

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[Braqzen]

I'm a little confused about your comments in the issue regarding the usage of a String and a StorageString. The type in the map is StorageString but you create a string that gets assigned to some arbitrary key and then via magic it becomes the StorageString type.

I've not looking into handling StorageKeys so there's probably something going on there.