Optimize AES-CTR encryption/decryption

[kixelated]

Much of the time spent is performing cipher.NewCTR for each packet.
This function allocates ~600 bytes on the heap each time which is a
little excessive to encrypt 1000 bytes.

I added a Reset function that allows cipher reuse. In addition, I sped
up the CTR cipher taking advantage of the fixed aes.BlockSize.

Unfortunately, forking the Go CTR implementation requires xorBytes,
which is not exported. There's a Go issue open that will hopefully
export the function because it's pretty useful. Until then, there needs
to be a lot of copy-pasted assembly code related to fast XOR bytes.

golang/go#30553

name                 old time/op   new time/op   delta
EncryptRTP-8          3.66µs ± 6%   3.36µs ± 6%   -8.04%  (p=0.001 n=10+10)
EncryptRTPInPlace-8   3.38µs ± 8%   3.13µs ± 5%   -7.33%  (p=0.000 n=10+10)
DecryptRTP-8          3.69µs ± 7%   3.37µs ± 8%   -8.80%  (p=0.000 n=10+10)
Write-8               3.80µs ± 9%   3.45µs ± 5%   -9.33%  (p=0.000 n=10+10)
WriteRTP-8            3.72µs ± 7%   3.46µs ± 8%   -6.96%  (p=0.005 n=10+10)

name                 old speed     new speed     delta
EncryptRTP-8         277MB/s ± 6%  301MB/s ± 6%   +8.76%  (p=0.001 n=10+10)
EncryptRTPInPlace-8  300MB/s ± 7%  323MB/s ± 5%   +7.85%  (p=0.000 n=10+10)
DecryptRTP-8         277MB/s ± 7%  304MB/s ± 7%   +9.68%  (p=0.000 n=10+10)
Write-8              266MB/s ± 8%  294MB/s ± 5%  +10.24%  (p=0.000 n=10+10)
WriteRTP-8           272MB/s ± 7%  293MB/s ± 8%   +7.61%  (p=0.005 n=10+10)

[kixelated]

Let me know if you would prefer that I just fork pions/srtp instead this approach.

[kixelated]

Kind of an overkill approach.