More units for the P/Invoke tool

In a handful of places, my P/Invoke command line tool accepts a unit for a size, so allocslot can take a size measured in chars rather than bytes. It might be helpful, though, to measure size using any kind, e.g. when allocating an array. To allow that, I added two keywords to the unit parser function. sizesof takes the name of a kind (e.g. native). This makes chars somewhat unnecessary since sizesof short will work the same, but chars is more memorable and one of the more common expected units, so it's reasonable to have a shortcut. blockssizedlike takes the name of a slot and looks at its kind; this is helpful for working custom structures defined with block.