Allow Array#join to allocate smaller strings

rb_str_buf_new always allocates at least 127 bytes of capacity, even
when less is requested.

    > ObjectSpace.dump(%w[a b c].join)
    {"address":"0x7f935f06ebf0", "type":"STRING", "class":"0x7f935d8b7bb0", "bytesize":3, "capacity":127, "value":"abc", "encoding":"UTF-8", "memsize":168, "flags":{"wb_protected":true}}

Instead, by using rb_str_new and then setting the length to 0, we can
allocate the exact amount of memory needed, without extra capacity.

    > ObjectSpace.dump(%w[a b c].join)
    {"address":"0x7f903fcab530", "type":"STRING", "class":"0x7f903f8b7988", "embedded":true, "bytesize":3, "value":"abc", "encoding":"UTF-8", "memsize":40, "flags":{"wb_protected":true}}

0 files changed, 0 insertions, 0 deletions