Fix incorrect assertion when deallocating big block

Fix incorrect assertion when deallocating big block

Since a big_block rounds up the size to a multiple of big_block::min it
is wrong to assert that the supplied number of bytes equals the
big_block's size(). Add big_block::alloc_size(size_t) to calculate the
allocated size consistently, and add comments to the code.

	* src/c++17/memory_resource.cc (big_block): Improve comments.
	(big_block::all_ones): Remove.
	(big_block::big_block(size_t, size_t)): Use alloc_size.
	(big_block::size()): Add comment, replace all_ones with equivalent
	expression.
	(big_block::align()): Shift value of correct type.
	(big_block::alloc_size(size_t)): New function to round up size.
	(__pool_resource::allocate(size_t, size_t)): Add comment.
	(__pool_resource::deallocate(void*, size_t, size_t)): Likewise. Fix
	incorrect assertion by using big_block::alloc_size(size_t).
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc: Add
	more tests for unpooled allocations.

From-SVN: r266088