PERF: faster placement creating extension blocks from arrays
When creating a DataFrame from many arrays stored in ExtensionBlocks, it seems quite some time is taken inside BlockPlacement using `np.require` on the passed list. Specifying the placement as a slice instead gives a much faster creation of the BlockPlacement. This delays the conversion to an array, though, but afterwards the conversion of the slice to an array inside BlockPlacement when neeeded is faster than an initial creation of a BlockPlacement from a list/array of 1 element.

From investigating https://github.com/pandas-dev/pandas/issues/32196#issuecomment-600824238

@rth this reduces it with another third! (only from the dataframe creation, to be clear)