array implementation: several small changes

src/array.c:
 - make various functions without declaration in the mruby headers static
 - removed all uncessary int casts and two useless comments
 mrb_ary_new_from_values: move to similar functions
 mrb_check_array_type: fix indentation

include/mruby/array.h:
 mrb_shared_array: padding (default "mrbconf.h" on an usual 64-bit system)
                   sizeof(mrb_int)==sizeof(int)==4 && sizeof(mrb_value*)==8

both:
 mrb_ary_aget:  remove declaration in header and make static
                nobody uses it which is not surprising since it has 1 req arg!
 mrb_assoc_new: small optimization and move to similar functions
 mrb_ary_len:   re-implement as static inline function (it is used by mrbgems)
                programmers should directly use RARRAY_LEN instead
Merge pull request #1697 from cremno/array-changes

array implementation: several small changes