Remove constant allocations from AEAD_AES_GCM

[agnivade]

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

[codecov]

Codecov Report

Merging #170 (dff36f5) into master (eda10ea) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #170   +/-   ##
=======================================
  Coverage   76.12%   76.12%           
=======================================
  Files          16       16           
  Lines         775      775           
=======================================
  Hits          590      590           
  Misses        103      103           
  Partials       82       82           

Flag	Coverage Δ
go	76.12% <100.00%> (ø)
wasm	75.61% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
srtp_cipher_aead_aes_gcm.go	70.73% <100.00%> (ø)

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