Ensure common short functions get inlined

[randoragon]

This must happen for all O(1) getters such as sp_stack_peek and sp_stack_get. There should be NO reason or incentive to worry about loops such as

for (size_t i = 0; i < stack->size; i++) {
    const int elem = sp_stack_geti(stack, i);
    /* do something with elem */
}

generating a function call every iteration, which is (probably) way slower than accessing stack->data directly. This should be trivially guaranteed.

I'm aware C89 does not support enforcing inlining. But most compilers support attributes which allow this. Staple is and always will be compiler-agnostic, but adding some attributes guarded by compiler-detecting ifdefs should be no problem. A bigger problem might be moving function definitions to headers. In any case, this will require some significant modifications to the lua generate tool.

Also, before anything, a benchmark should be conducted to make sure forcing an inlining actually leads to better performance.