Add mem.secureEql

[lukechampine]

Constant-time comparisons prevent timing attacks in sensitive crypto code. For example, Go's poly1305 implementation uses a constant-time routine to verify MACs. Now Zig users can do the same with mem.secureEql. (But since this is easy to forget, it may be prudent to add a verify function to Zig's Poly1305 struct, and elsewhere.)

Currently this function only works on integer types. I imagine it could be made to work with other types as well, but I'm very new to Zig so I didn't attempt to do so.

[andrewrk]

This is unsound. See #1776 for an explanation and attempt to address this.

In summary the compiler is allowed to figure out that this is a memory comparison operation and replace the implementation with something that is not constant time.

Until #1776 is closed, the use case of constant time operations must be done with a third party library, inline assembly, or a heavy handed approach such as delaying the result until a fixed amount of time has transpired.

[lukechampine]

my bad, should have searched first. Looking forward to seeing how Zig solves this!