Overflow from integer negation expressions

[mattheww]

§6.5.4:18 (fls_CutpaCFCGHQs) includes:

If the result of the negation expression does not fit within the range of the operand type, then arithmetic overflow occurs.

There is an exception which needs to be documented here: arithmetic overflow doesn't occur in some cases where the operand includes a literal and the value of the expression would fit in the type (eg -128_u8 or -(128_u8)).

See rust-lang/reference#1188 for how this was addressed there.