runtime safety checking for pointer arithmetic

[andrewrk]

There should be a test in test/runtime_safety.zig to make sure that unknown length pointer arithmetic does not wrap past the end of the address space.

[andrewrk]

Extracted from https://github.com/ziglang/zig/issues/1059#issuecomment-462101897

• [ ] compile error test for compile time pointer arithmetic overflowing
• [ ] runtime safety test for pointer arithmetic overflowing
• [ ] compile error test for comptime ptr arithmetic with undefined
• [ ] docs

These apply to unknown length pointers and C pointers.

[wooster0]

Should also check that the resulting pointer if non-allowzero doesn't result in zero like it can currently:

const std = @import("std");

pub fn main() void {
    const x: [*]u8 = @ptrFromInt(1);
    const a: [*]u8 = x - 1;
    std.debug.print("address: {*}\n",.{a});
}

address: u8@0