Refactor blokpos from u16 to i16

[suprohub]

In new versions block can place on -y coords up to -60.

[birbe]

for backwards compatibility, the internal coordinate system's Y axis starts at 0, so if you're using the bigger chunk layout the range is [0, 384), which is trivial to convert between the coordinates using a negative number. also, it's up to -64 and not -60