Add optional support for AEGIS encryption

AEGIS is a new family of authenticated encryption algorithms that offers
stronger security, higher usage limits, and better performance than AES-GCM.

This pull request adds support for a new `-aegis` command-line flag, allowing
AEGIS-128X2 to be used as an alternative to AES-GCM on CPUs with AES acceleration.

It also introduces the ability to use ciphers with different key sizes.

More information on AEGIS is available here:
- https://cfrg.github.io/draft-irtf-cfrg-aegis-aead/draft-irtf-cfrg-aegis-aead.html
- https://github.com/cfrg/draft-irtf-cfrg-aegis-aead

gocryptfs -speed speed on Apple M1:

AES-GCM-256-OpenSSL              3718.79 MB/s
AES-GCM-256-Go                   5083.43 MB/s   (selected in auto mode)
AES-SIV-512-Go                    625.20 MB/s
XChaCha20-Poly1305-OpenSSL       1358.63 MB/s   (selected in auto mode)
XChaCha20-Poly1305-Go             832.11 MB/s
Aegis128X2-Go                   11818.73 MB/s

gocryptfs -speed speed on AMD Zen 4:

AES-GCM-256-OpenSSL              5215.86 MB/s
AES-GCM-256-Go                   6918.01 MB/s   (selected in auto mode)
AES-SIV-512-Go                    449.61 MB/s
XChaCha20-Poly1305-OpenSSL       2643.48 MB/s
XChaCha20-Poly1305-Go            3727.46 MB/s   (selected in auto mode)
Aegis128X2-Go                   28109.92 MB/s

1 files changed, 16 insertions, 0 deletions

diff --git a/internal/stupidgcm/aegis_test.go b/internal/stupidgcm/aegis_test.go
new file mode 100644
index 0000000..36ef763
--- /dev/null
+++ b/internal/stupidgcm/aegis_test.go
@@ -0,0 +1,16 @@
+//go:build !without_aegis && cgo
+// +build !without_aegis,cgo
+
+package stupidgcm
+
+import "testing"
+
+func TestStupidAegis(t *testing.T) {
+	if BuiltWithoutAegis {
+		t.Skip("Aegis support has been disabled at compile-time")
+	}
+	key := randBytes(16)
+	c := NewAegis(key)
+
+	testCiphers(t, c, c)
+}