There are two variants of ChaCha20 in use. The original one uses a 64-bit
nonce and a 64-bit block counter, while the one used in RFC8439 uses a
96-bit nonce and 32-bit block counter. This commit changes the interface
to use the 96/32 split (but automatically incrementing the first 32-bit
part of the nonce when the 32-bit block counter overflows, so to retain
compatibility with >256 GiB output).
Simultaneously, also merge the SetIV and Seek64 functions, as we almost
always call both anyway.
Co-authored-by: dhruv <856960+dhruv@users.noreply.github.com>
