Allow to preallocate space for blocks

[homonoidian]

In certain words we know the size of the resulting block ahead-of-time. To avoid reallocs on larger blocks, we could allow to preallcoate the resulting block. E.g. in map we know the size (although I don't quite remember).

A huge question is how to present such a thing. Having something like "( N -- B )" orphanFor would work, I suppose, with an infix version e.g. orphanFor: 100.

The block would still look empty, though. Idk if this is a good idea.