Use libraries over "system" commands

[eayus]

Currently a lot of our work is done using "system" commands. We should switch to libraries if they are available because:

• It's faster, since the command line arguments do not need to be processed.
• Easier to keep track of the "hidden" dependencies of sirdi.
• It's more secure. On the command line, every argument is a string. This makes it much harder to prevent issues such as code injection.

The biggest offenders for this are probably git commands and idris2 --build ..., since there seems to be existing Idris2 libraries for both of these things.

[eayus]

So on further inspection, the Idris git library (https://github.com/bigs/idris2-libgit2) doesn't support everything we need. We need ls-remote equivalent behaviour (i.e., in libgit, https://github.com/libgit2/libgit2/blob/main/examples/ls-remote.c) so that we can get the latest commit hash of the repo.

I'm thinking that perhaps a more sensible solution is to just write a few C wrappers in this project for git stuff. We only really need clone and ls-remote at the moment.