Fix incorrect aes_gcm constants used in xaes-256-gcm

[eligrubb]

Fixes #837.

Currently the max length constants for plaintext (P_MAX), associated data (A_MAX), and ciphertext (C_MAX) are not correct for XAES-256-GCM.

	Current	Correct
P_MAX	1 << 36	(1 << 36) - 32
A_MAX	1 << 36	(1 << 61) - 1
C_MAX	(1 << 36) + 16	(1 << 36) - 32

This PR resolves this by using the aes_gcm crates constants instead of redefining them in this crate. Relying on the definition in aes_gcm makes sense because the underlying call to aes_gcm dictates the maximums for plaintext, aad, and ciphertext.

Additionally, the code previously included the 16-byte tag in the C_MAX length. However, the AeadInOut trait explicitly operates in a detached state where the tag is handled separately from the ciphertext, meaning the ciphertext is always the same length as the plaintext. So the current C_MAX check in decrypt_inout_detached() can be replaced with a P_MAX check (alternatively, C_MAX could be defined as equivalent to P_MAX).

All tests run and pass.