proper compilation of lib_lowlevel is confusing

[ctb]

I've been having trouble with developing FFI methods, and I think I figured out at least one of the problems (although I am not sure where to go for the solution).

In brief, the following doesn't work:

• add FFI function to Rust layer under src/core/src/ffi;
• make to update Rust shared library;
• run cbindgen with make include/sourmash.h;

Instead I seem to need to:

• add FFI function to Rust layer under src/core/src/ffi;
• make to update Rust shared library;
• run cbindgen with make include/sourmash.h;
• change the Rust FFI source code so that the Rust shared lib will be rebuilt;
• re-run make

It smells like maybe the Rust shared lib in some way depends on include/sourmash.h?

This is also on my Mac OS X laptop, so maybe there is a problem with .dylib vs .so in some dependency rule, or something.

On a somewhat related note, I am unsure how to rebuild the sourmash FFI layer properly without running make which runs pip install -e. Is there a maturin command I could be running?