fix wrong offset for pack_32 and pack_64 with repeat, closes #7

fix wrong offset for unpack number values with repeat, closes #6

rename `calcsize` to `computeLength`, and expose it. closes #5

updates for Nim 0.20.2, fix unpack -1 as reported in #4

compatible fixes for Nim devel version

more tweaks

various tweaks, new way to packing data