feat: Initial example of C integration

[jsantell]

Need to figure out some text encoding stuff and clean ups, but this C snippet links and runs against libnoosphere

[jsantell]

Now working. Some areas of improvements could be tying it into the larger build system:

• maybe lives in the noosphere crate?
• make is naive here as it's not rebuilding rust if it changes.
• Leak detection

Initial reactions to working with the raw FFI:

• Mostly copied the Swift tests, s/nil/NULL, and added semi-colons and it was smooth sailing other than remembering C trivia
• The generic wrapper types (slice_ref_uint8_t, slice_boxed_char_ptr_t) tripped me up, learning how safer_ffi maps Rust to C, initially criticizing the non-pointer args, but going back on that now. A small win here could be possibly renaming the generated types: slice_boxed_char_ptr_t -> ns_string_array_t, slice_boxed_uint8_t -> ns_bytes -- matching their corresponding free functions. But this challenge extends to all Rust-owned primitive types. Need to think more about this, wonder how other libraries handle this.
• The similarly named slice_boxed_uint8_t and slice_ref_uint8_t caused some confusion as well, with a slight difference in pointer types (ref being the const version) -- the "ref" type is for (from Rust's perspective) unowned data, used in args, and the "boxed" type is for return values. To simplify these intermediate types, maybe we can just use boxed for return values, and use two args (array, length) where needed (only ns_sphere_fs_write uses the ref type currently).
• Idiomatically, would love NsSphereFs_t -> ns_sphere_fs_t! Need to investigate feasibility of doing this in safer_ffi (meta issue: Customizable naming templates (e.g., namespace prefixing) getditto/safer_ffi#93)
• I had to link libmath (-lm clang/gcc arg) to get Noosphere linked, but that seems to be the only dependency

[cdata]

I love that we can add C to our retinue of supported languages. Some general feedback:

Can we put the example in a sub-folder? e.g., $ROOT/c/example/main.c

And, can we add a README.md at $ROOT/c/README.md? It doesn't have to be very long, but otherwise model it roughly after the other READMEs in our language folders.

Super cool :D

[cdata]

Oh, also: please file an issue to get a test workflow in place for CI. It would be nice to be made aware of conspicuous support regressions.

[jsantell]

• Added HeaderTransformer to transform generated header to C idiomatic types: NsSphereFs_t to ns_sphere_fs_t. Makes it possible to programmatically tag classes with correct Doxygen tags.
• Added explicit type casts between uint8_t* and char* to satiate clang (and actually try out both clang and gcc)
• Moved files around to make c/ a root language directory like others
• Filed Setup CI tests for C bindings #245

[cdata]

The transformer is very interesting 😳 we'll have to keep an eye on it to make sure we don't end up with any funky transformations.