Implement AES ECB, CBC and GCM modes

[qmuntal]

Please review #3 first.

This PR implements AES ECB, CBC and GCM modes. Each mode is implemented in its own commit.

CRT mode is not supported by CNG so it has not bee implemented, see #4.

The AES GCM TLS mode has been implemented in the same way as OpenSSL: microsoft/go-crypto-openssl#21 as CNG does not have a specific TLS mode.

[dagood]

I think there's one small fix, but otherwise LGTM.

[jaredpar]

Won't int(ret) potentially truncate on 32 bit platforms?

[qmuntal]

ret is a uint32, so I don't think it will overflow. What will happen if len(src) don't fit in 32 bits is that this function will panic. I guess this behavior is right, at least I won't feel comfortable batching encrypts/decrypts in 32 bit chunks unless a crypto expert says we can do it.

[dagood]

Another thing to look at is signed values: https://go.dev/play/p/ZQXsT1GlEPQ

	var v uint64 = math.MaxUint64 // 64 to match platform-sized int in playground
	fmt.Printf("%x %x\n%x\n", v, int(v), math.MaxInt)

ffffffffffffffff -1
7fffffffffffffff

But I think it's ok here, because int is 32-bit, and len returns an int, then the size of the slice is limited to the positive 31 bits and this will never occur. If bcrypt.Decrypt is misbehaving and returns a large value that casts to a negative int, that will always panic because len(src) always returns a positivenon-negative int. (And we want a panic in this case.)