What does HackerNews think of libhydrogen?
A lightweight, secure, easy-to-use crypto library suitable for constrained environments.
You can use includes to have "a single file to add to the project", and still split your code in smaller, readable files.
See https://github.com/jedisct1/libhydrogen for example. It's a single-file crypto library from an application perspective, but everything was not stored into a single, hard-to-navigate file.
Both the API and the underlying primitives are still moving targets, but one of the motivations behind this project was to revisit the NaCl/LibSodium/SUPERCOP API to build something more difficult to misuse.
In particular, users never provide nonces. Contexts are mandatory everywhere. A broken PRG is not always catastrophic. Operations requiring keys have a dedicated keygen() function (eventually, keys may have their own type instead of generic byte buffers). There's no crypto_stream_* API, but a randombytes_buf_deterministic() function.
libhydrogen is not meant to replace libsodium, but the libsodium2 API may resemble more to it than the current one.