The `rb_fstring(rb_enc_str_new())` pattern is inneficient because:

- It passes a mutable string to `rb_fstring` so if it has to be interned
  it will first be duped.
- It an equivalent interned string already exists, we allocated the string
  for nothing.

With `rb_enc_interned_str` we either directly get the pre-existing string
with 0 allocations, or efficiently directly intern the one we create
without first duping it.