Fix redundant API syntax

The "value" key is excessive, one can just feed the value!

curl -X POST http://localhost:3000/lists/mylist/items \
     -d '{"value": "item1"}' -H "Content-Type: application/json"

curl -X PUT http://localhost:3000/lists/mylist/0 \
     -d '{"value": "item1"}' -H "Content-Type: application/json"

curl -X PUT http://localhost:3000/dicts/mydict/mykey \
     -d '{"value": "value1"}' \
     -H "Content-Type: application/json"

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Side note: #3 (closed) is solved, but I could consider this later:

use serde_json::Value;

pub type SharedVectors = Arc<Mutex<HashMap<String, Vec<Value>>>>;
pub type SharedHashMaps = Arc<Mutex<HashMap<String, HashMap<String, Value>>>>;

I'm on the fence about it, though. I see some limits in what I do now as an advantage and I don't want dig Serde deeply yet.