Function to_indexable_string 

pub fn to_indexable_string(v: &Value) -> String

Encode a JSON value into a string that sorts lexicographically in CouchDB collation order. Used as keys in the storage engine.

Format:

• Null: "1"
• Bool: "2F" / "2T"
• Number: "3" + encoded number
• String: "4" + string value
• Array: "5" + encoded elements separated by null byte
• Object: "6" + encoded key-value pairs