ngtcp2 for Nim

Wrapper around the ngtcp2 C library for Nim.

Updating to a newer version

Follow these steps when updating the wrapper to a newer version of ngtcp2:

update the git submodule in libs/ to point to the new version
- git submodule update --init --recursive
run build.sh (requires Nim, CMake and clang to be installed)
increase the version property in the ngtcp2.nimble file
commit the changes