Add better support for non-Flatware procedures #183

Writing a non-Flatware program is pretty rough right now; a lot of general-purpose functions (e.g., copying from an ro/rw memory to program memory) seem to only be implemented in Flatware, and need to be separately implemented and linked to non-Flatware programs. Possibly related to #40, although that's more specific. Ideally we'd also have helpers for higher-level operations (like "create blob from char * and size_t").

Also just generally the build process is pretty hard to get right, and returns cryptic errors if it's done wrong (e.g., the output of wasm2c failing to compile because wasm2c_function_instantiate was passed the wrong number of arguments).