Rtwobitlib is an R package that contains a trimmed down copy of the kent-core source tree turned into a C library for manipulation of .2bit files. See https://genome.ucsc.edu/FAQ/FAQformat.html#format7 for a quick overview of the 2bit format.
The kent-core source tree can be found here. Only the .c and .h files from the source tree that are related to manipulation of .2bit files were kept.
Note that the package is primarily useful to developers of other R packages who wish to use the 2bit C library in their own C/C++ code.