std.math: Add std.math.egcd to compute the extended GCD

This adds an iterative EGCD implementation to std.math.

The implementation is simple and conservative, returns Bézout
coefficients consistent with widely‐used implementations, and has
been formally verified correct for any value and type supported
by Zig up to u65534/i65534.

Its purpose is to make this API available to projects and other
parts of the standard library. Optimizations for specific types or
value ranges (e.g., using a binary method or approximations) can be
added later without any changes to existing applications.

Author: jedisct1

lib/std/math.zig

@@ -237,6 +237,7 @@ pub const atanh = @import("math/atanh.zig").atanh;
 pub const sinh = @import("math/sinh.zig").sinh;
 pub const cosh = @import("math/cosh.zig").cosh;
 pub const tanh = @import("math/tanh.zig").tanh;
+pub const egcd = @import("math/egcd.zig").egcd;
 pub const gcd = @import("math/gcd.zig").gcd;
 pub const lcm = @import("math/lcm.zig").lcm;
 pub const gamma = @import("math/gamma.zig").gamma;
@@ -398,6 +399,7 @@ test {
     _ = sinh;
     _ = cosh;
     _ = tanh;
+    _ = egcd;
     _ = gcd;
     _ = lcm;
     _ = gamma;