crypto: Implement RFC8439-compatible variant of ChaCha20

Summary:
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>

tests: improve ChaCha20 unit tests

test: add ChaCha20 test triggering 32-bit block counter overflow

Verify that our ChaCha20 implementation using the 96/32 split interface
is compatible with >256 GiB outputs by triggering a 32-bit block counter
overflow and checking that the keystream matches one created with an
alternative implementation using a 64/64 split interface with the
corresponding input data. The test case data was generated with the
following Python script using the PyCryptodome library (version 3.15.0):

----------------------------------------------------------------------------------------------
from Crypto.Cipher import ChaCha20
key = bytes(list(range(32))); nonce = 0xdeadbeef12345678; pos = 2**32 - 1
c = ChaCha20.new(key=key, nonce=nonce.to_bytes(8, 'little'))
c.seek(pos * 64); stream = c.encrypt(bytes([0])*128)
print(f"Key: {key.hex()}\nNonce: {hex(nonce)}\nPos: {hex(pos)}\nStream: {stream.hex()}")
----------------------------------------------------------------------------------------------

This is a backport of core#27985
Depends on D18827

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18828