Core: add SignedNumeric protocol

[MaxDesiatov]

Depends on #55.

I'm not adding the default (and only) implementation here yet, as it crashes the type checker with Assertion failed: (protocol && "requirements should have stopped recursion") error message:

extension SignedNumeric {
  @_transparent
  public static prefix func - (_ operand: Self) -> Self {
    var result = operand
    result.negate()
    return result
  }

  @_transparent
  public mutating func negate() {
    self = 0 - self
  }
}

[compnerd]

Interesting, sounds like we are missing some requirements that the compiler expected? We should try to identify what those requirements are at the very least before we merge this I think.

[MaxDesiatov]

A lot of numerical stuff requires a huge amount of operators and their dependencies introduced at once. I had to omit some of those to split it into separate PRs. Would you find it acceptable if I don't omit anything and submit things without breaking these cyclic dependencies? That would result in huge PRs.

[compnerd]

I think that if we know/understand what is missing and having that as FIXMEs is fine. We need to break the dependency cycle somewhere and I think that doing that iteratively is going to be easier.

[MaxDesiatov]

Main blocker for me here is associatedtype Words: RandomAccessCollection on BinaryInteger, and both of those depend on Strideable, which depends on SignedNumeric. I could add a FIXME in this PR, saying that default implementation for negate() and prefix func - is missing. Would that be ok?

[compnerd]

Perfet! At least we understand what is missing and why, so I don't see that as a blocker for making progress.

[MaxDesiatov]

Great, that's ready now.