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.