Implement SortedSet key-value entry type

[ecton]

Closes #145

Proposed API

Sorted Set Entrypoint:

• db.sorted_set("key"): Returns a builder.

Functions available on the sorted-set builder when no range has been provided:

• .await: Returns the entire sorted set as a SortedSet. Equivalent to db.get_get("key").into().await
• .range(impl RangeBounds<Score>): Returns a new builder instance that has a range, changing the APIs that are available.
• insert(Value, Score).await: Inserts a new entry. Returns Option<Score>.
• increment_by(Value, Score).await: Increments the score for the value by the score passed in. Returns the new Score.
• decrement_by(Value, Score).await: Same as above but decrement.
• score(Value).await: Returns the score for the value. Option<Score>
• index_of(Value).await: Returns the index of the value as sorted by the scores. Option<u64>.
• delete(Value).await: Deletes the value. Returns the score if one existed: Option<Score>.

Functions available on the sorted-set builder with or without a range provided:

• values().await: Returns only the values: Vec<Value>
• scores().await: Returns only the scores: Vec<Score>
• count().await: Returns the number of value: u64
• pop_front().await: Removes and returns the first value/score pair combination: Option<(Value, Score)>.
• pop_back().await: Removes and returns the last value/score pair combination: Option<(Value, Score)>.
• clear().await: Deletes the matching values.