Add precondition messages

[oscbyspro]

I suppose precondition messages are nice to have. I would consider it a small quality-of-life improvement, for when things go wrong. Looking at [U]Int128 (#109), the "overflow in [x]" precondition message format seems descriptive enough.

[oscbyspro]

I suppose messages are only really useful for programmer inputs.

assert(2 < 3, "something is broken") // useless message

[oscbyspro]

Something like this might be a bit more maintainable:

precondition(!overflow, callsiteOverflowInfo())

func callsiteOverflowInfo(function: StaticString = #function, file: StaticString = #file, line: UInt = #line) -> String {
    "overflow in \(function) at \(file):\(line)"
}