core/crypto: Add x25519

This package implements the X25519 key agreement scheme as specified in
RFC 7748, using routines taken from fiat-crypto and Monocypher.

core/crypto/_fiat/fiat.odin

@@ -0,0 +1,24 @@
+package fiat
+
+// This package provides various helpers and types common to all of the
+// fiat-crypto derived backends.
+
+// This code only works on a two's complement system.
+#assert((-1 & 3) == 3)
+
+u1 :: distinct u8
+i1 :: distinct i8
+
+cmovznz_u64 :: #force_inline proc "contextless" (arg1: u1, arg2, arg3: u64) -> (out1: u64) {
+	x1 := (u64(arg1) * 0xffffffffffffffff)
+	x2 := ((x1 & arg3) | ((~x1) & arg2))
+	out1 = x2
+	return
+}
+
+cmovznz_u32 :: #force_inline proc "contextless" (arg1: u1, arg2, arg3: u32) -> (out1: u32) {
+	x1 := (u32(arg1) * 0xffffffff)
+	x2 := ((x1 & arg3) | ((~x1) & arg2))
+	out1 = x2
+	return
+}