introduce ZALLOC{,_N} to replace ALLOC{,_N}+MEMZERO use

Using calloc where possible reduces code and binary sizes.

* include/ruby/ruby.h (ZALLOC, ZALLOC_N): implement
  (Data_Make_Struct, TypedData_Make_Struct):
  ZALLOC replaces ALLOC+memset
* compile.c (iseq_seq_sequence): ZALLOC_N replaces ALLOC_N+MEMZERO
* cont.c (fiber_t_alloc): ZALLOC replaces ALLOC+MEMZERO
* io.c (rb_io_reopen): ditto
* iseq.c (prepare_iseq_build): ditto
* parse.y (new_args_tail_gen, parser_new, ripper_s_allocate): ditto
* re.c (match_alloc): ditto
* variable.c (rb_const_set): ditto
* ext/socket/raddrinfo.c (get_addrinfo): ditto
* ext/strscan/strscan.c (strscan_s_allocate): ditto
* gc.c (rb_objspace_alloc): calloc replaces malloc+MEMZERO

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e