provide ##core#inline-routines for cheap SRFI-4 vector primitives

[felix winkelmann]

XXXvector-length, XXXvector-ref and XXXvector-set! would be much more efficient if routines callable via ##core#inline[_allocate] would be available for them (just as it currently is for, say, vector-ref). Since the structure-tag is not available in the C runtime, it has to be explicitly passed, e.g.

C_word C_i_XXXvector_ref(C_word tag, C_word vec, C_word index);
C_word C_i_XXXvector_set(C_word tag, C_word vec, C_word index, C_word x);
C_word C_i_XXXvector_length(C_word tag, C_word vec);