Smaller id_table on 64bit platform
Reduce size of id_table by 25% on 64bit platform (19% on smallest table):
- embed collision bit into key
- use array of keys + array of values instead of array of struct item_t. Both arrays allocated at once, and pointer to values is calculated.
- stricter check in next_id_base.
One bit is needed for collision check on 64bit platform.
Range is more limited on 32bit platform with ID_SCOPE_SHIFT and RUBY_SPECIAL_SHIFT.
- stricter check for result of next_id_base.
Rais exception in rb_sym2id, assert in rb_make_internal_id.
- allow to shrink id_table.
Shrink was disabled for mix_table cause it leads to error in id_table type
detection. Since mix_table were removed, it is safe to shrink it again.
- improve large table performance by mixin higher bits into step value.
Performance of small tables almost the same.
Mixing higher bits takes 1-2% for small tables on hit, but improves large table