Symbols deserialize incorrectly.

[bolli24]

When deserializing symbols from eg. JSON the resulting values are off by one.

Example:

use string_interner::{DefaultSymbol, StringInterner};

fn main() {
    let mut interner = StringInterner::default();
    let symbol = interner.get_or_intern("hello");

    let serialized = serde_json::to_string(&symbol).unwrap();
    println!("Original: {:?}, JSON: {}", symbol, serialized);

    let deserialized: DefaultSymbol = serde_json::from_str(&serialized).unwrap();
    println!("Deserialized: {:?}", deserialized);

    match interner.resolve(deserialized) {
        Some(s) => println!("Resolves to: {}", s),
        None => println!("Cannot resolve symbol!"),
    }
}

Expected Output:

Original: SymbolU32 { value: 1 }, JSON: 1
Deserialized: SymbolU32 { value: 1 }
Resolves to: hello

Actual Output

Original: SymbolU32 { value: 1 }, JSON: 1
Deserialized: SymbolU32 { value: 2 }
Cannot resolve symbol!

[caspark]

Will be fixed by #91 if/when it's merged.