Desired capacity for each nodes

By default at 32, it can be increased for very large lists.

It can be modified anytime, but newly created nodes may still be assigned the same capacity of old nodes when created by insert.

Property definitions

more_collections $ UnrolledList :: nodes_length
	# Desired capacity for each nodes
	#
	# By default at `32`, it can be increased for very large lists.
	#
	# It can be modified anytime, but newly created nodes may still be assigned
	# the same capacity of old nodes when created by `insert`.
	var nodes_length = 32 is writable
lib/more_collections/more_collections.nit:361,2--367,34