update CRC RevEng links in README.md

improve handling of unbounded -b slices (#9)

make definition of BITS_TO_LIMBS less confusing

update unit tests from RevEng CRC catalogue

brace expansion support for -b switches

support negative values in bigint_from_string()

fix error message format specifier for bitsize_t

improve ./check.sh code coverage

fix initial allocation of slice structures

refactor the code to make it more understandable

add missing bittypes.h file

fix l:r slices where l < 0 and relative r >= -l

better error messages for invalid -o/-O offsets

improve forge() gaussian elimination algorithm

define bitsize_t and bitoffset_t types