Simplify naming in i257

[delaaxe]

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the new behavior?

While code like the following is eventually understandable, it isn't as clearly obvious as the proposed one:

Before:

fn foo(value: i257) {
  let mut x: u256 = 42;
  if value.sign == true {
    x += value.inner;
  }
}

After:

fn foo(value: i257) {
  let mut x: u256 = 42;
  if value.is_negative {
    x += value.abs;
  }
}

Does this introduce a breaking change?

• Yes
• No

Other information

[0xLucqs]

@shramee any opinion ?

[shramee]

Yeah, I think is_negative makes it clearer! ❤️

[delaaxe]

If you're ok I'd also do

struct i257 {
    abs: u256,
    is_negative: bool,
}

instead of

struct i257 {
    is_negative: bool,
    abs: u256,
}

as it would make storage packing a bit more intuitive if ever needed