Optimize data types of internal structs

[hayashi-mas-wap]

We have a lot data type conversions:
e.g.

sudachi.rs/sudachi/src/dic/lexicon_set.rs

Line 102 in 440ff7d

lexicon.set_dic_id(self.lexicons.len() as u8);

sudachi.rs/sudachi/src/dic/lexicon_set.rs

Line 179 in 440ff7d

let dict_id = id.dict().as_raw() as usize;

Most of them are necessary, but we may optimize how to keep values in each structs to reduce type conversion.

As this relates to the performance, we need to measure run time.