We remove all cases of constant allocations with an array
declaration which makes all of them to be allocated on stack instead.

Before:

```
./srtp_cipher_aead_aes_gcm.go:22:7: &srtpCipherAeadAesGcm{} escapes to
heap
./srtp_cipher_aead_aes_gcm.go:151:12: make([]byte, 12) escapes to heap
./srtp_cipher_aead_aes_gcm.go:73:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:94:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:170:12: make([]byte, 12) escapes to heap
./srtp_cipher_aead_aes_gcm.go:111:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:114:42: make([]byte, 12) escapes to heap
./srtp_cipher_aead_aes_gcm.go:131:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:134:42: make([]byte, 12) escapes to heap
./srtp_cipher_aead_aes_gcm.go:187:13: make([]byte, 12) escapes to heap
```

After:

```
./srtp_cipher_aead_aes_gcm.go:22:7: &srtpCipherAeadAesGcm{} escapes to
heap
./srtp_cipher_aead_aes_gcm.go:73:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:94:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:111:22: make([]byte, size) escapes to heap
./srtp_cipher_aead_aes_gcm.go:131:22: make([]byte, size) escapes to heap
```