Standardize type U256 and Word

[hero78119]

Describe the feature you would like

After introduce word lo/hi optimisation, now there are 2 ambiguous Word type.

• eth_types::Word
• utils::word::Word<F>

Proposed by @adria0, since eth_types::Word just a type alias of U256, we might consider remove it and turn to use U256 instead. See discussion thread https://github.com/privacy-scaling-explorations/zkevm-circuits/pull/1414#issuecomment-1557430722

Additional context

No response

[ed255]

We also have the trait ToWord with the method to_word. If we remove Word we can rename the trait and method to ToU256, to_u256.

[ChihChengLiang]

I want to add that using U256 also solves a problem in the Keccak circuit. We use the term "word" for the u64 word in the Keccak circuit, which confuses with the EVM U256 word.