optimized small functions that can be directly called

[gerdstolpmann]

As an extension of #2:

A second function definition is provided for small functions that can be directly called. This second definition passes one OCaml arg per Wasm arg:

function letrecX_direct(i32 envptr, i32 param1, i32 param2, ...) : i32

This shall only be done if:

• there are no allocations
• the number of args is small
• the size of the function body is small
• no incompatible features like APPLY and APPTERM

Also, note that fp is unavailable. So nothing can be stored on the stack. Because there are no allocations, all intermediate values can be put into local variables.