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 #1059 (comment)

• 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.