While profiling msgpack-ruby I noticed a very substantial amout of time spent in `rb_enc_associate_index`, called by `rb_utf8_str_new`. On that benchmark, `rb_utf8_str_new` is 33% of the total runtime, in big part because it cause GC to trigger often, but even then `5.3%` of the total runtime is spent in `rb_enc_associate_index` called by `rb_utf8_str_new`. After closer inspection, it appears that it's performing a lot of safety check we can assert we don't need, and other extra useless operations, because strings are first created and filled as ASCII-8BIT and then later reassociated to the desired encoding. By directly allocating the string with the right encoding, it allow to skip a lot of duplicated and useless operations. After this change, the time spent in `rb_utf8_str_new` is down to `28.4%` of total runtime, and most of that is GC.