Update mpsc padding

[mratsim]

This updates the MPSC channel padding after the fix of nim-lang/Nim#13122

There is a tricky compromise for the pledges data structure size:

weave/weave/channels/pledges.nim

Lines 146 to 173 in 71cf681

	PledgeKind = enum
	Single
	Iteration
	PledgePtr = ptr object
	refCount: Atomic[int32]
	case kind: PledgeKind
	of Single:
	impl: PledgeImpl
	of Iteration:
	numBuckets: int32
	start, stop, stride: int32
	impls: ptr UncheckedArray[PledgeImpl]
	PledgeImpl = object
	# Issue: https://github.com/mratsim/weave/issues/93
	# TODO, the current MPSC channel always use a "count" field.
	# Contrary to StealRequest and the remote freed memory in the memory pool,
	# this is not needed, and atomics are expensive.
	# It can be made optional with a useCount static bool.
	# The MPSC Channel is intrusive to the PledgeImpl.
	# The end fields in the channel should be the consumer
	# to avoid cache-line conflicts with producer threads.
	chan: ChannelMpscUnboundedBatch[TaskNode]
	deferredIn: Atomic[int32]
	deferredOut: Atomic[int32]
	fulfilled: Atomic[bool]

The kind field (1 byte) has to be the first field of PledgePtr, so when the ChannelMPSCunbounded[TaskNode] is made intrusive, the cacheline alignment requirements automatically mades it take over 64 bytes.
Also even if the back/count/front are tagged with 64 bytes alignment each (total 192 bytes)

weave/weave/channels/channels_mpsc_unbounded_batch.nim

Lines 50 to 80 in 71cf681

	ChannelMpscUnboundedBatch*[T: Enqueueable] = object
	## Lockless multi-producer single-consumer channel
	##
	## Properties:
	## - Lockless
	## - Wait-free for producers
	## - Consumer can be blocked by producers when they swap
	## the tail, the tail can grow but the consumer sees it as nil
	## until it's published all at once.
	## - Unbounded
	## - Intrusive List based
	## - Keep an approximate count on enqueued
	## - Support batching on both the producers and consumer side
	# TODO: pass this through Relacy and Valgrind/Helgrind
	# to make sure there are no bugs
	# on arch with relaxed memory models
	# Producers and consumer slow-path
	back{.align: MpscPadding.}: Atomic[pointer] # Workaround generic atomics bug: https://github.com/nim-lang/Nim/issues/12695
	# Accessed by all
	count: Atomic[int]
	# Consumer only - front is a dummy node
	front{.align: MpscPadding.}: derefMPSC(T)
	# back and front order is chosen so that the data structure can be
	# made intrusive to consumer data-structures
	# like the memory-pool and the pledges so that
	# producer accesses don't invalidate consumer cache-lines
	#
	# The padding is a compromise to keep back and front 1 cache line apart
	# but still have a reasonable MPSC Channel size of 2xCacheLine (instead of 3x)

the extra deferredIn, deferredOut, fulfilled fields are not using the extra space after the front field:

weave/weave/channels/pledges.nim

Lines 160 to 173 in 71cf681

	PledgeImpl = object
	# Issue: https://github.com/mratsim/weave/issues/93
	# TODO, the current MPSC channel always use a "count" field.
	# Contrary to StealRequest and the remote freed memory in the memory pool,
	# this is not needed, and atomics are expensive.
	# It can be made optional with a useCount static bool.
	# The MPSC Channel is intrusive to the PledgeImpl.
	# The end fields in the channel should be the consumer
	# to avoid cache-line conflicts with producer threads.
	chan: ChannelMpscUnboundedBatch[TaskNode]
	deferredIn: Atomic[int32]
	deferredOut: Atomic[int32]
	fulfilled: Atomic[bool]

probably because the data structures are distinct.

This makes it impossible to have PledgePtr[] be under 256 bytes. The 3 workarounds are:

1. Increase WV_MemBlockSize to 512 bytes
2. Don't have PledgeImpl intrusive to PledgePtr. This requires an extra allocation/pointer indirection
3. Reduce the size taken by MPSC channels

Given that before nim-lang/Nim#13122, there was no padding and the MPSC channel could reasonably handle cache line invalidation and that in most MPSC channels implementations there is no padding at all, solution 3 is chosen in this PR