Creating -unsafe dependant array/string/bigarray access primitives

After discussion with @planar and @gasche, here is a small patch that adds some new primitives to the compiler.

Basically %array_switched_get behaves as %array_safe_get unless the -unsafe option is activated, in which case it behaves as %array_unsafe_get.

It is inspired from the last commit by @Octachron (see #69) and may help remove the syntactic trick used for array/string/ba access.