Make overflowing and wrapping negation const
Make overflowing and wrapping negation const

Remember that the signed and unsigned versions are slightly different here, so there's four functions made const instead of just two.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust