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.

[Bug #20162]

Creating a ST table then calling st_replace leaks memory because the
st_replace overwrites the ST table without freeing any of the existing
memory. This commit changes it to use st_copy instead.

For example:

    RubyVM::Shape.exhaust_shapes

    o = Object.new
    o.instance_variable_set(:@a, 0)

    10.times do
      100_000.times { o.dup }

      puts `ps -o rss= -p #{$$}`
    end

Before:

    23264
    33600
    42672
    52160
    61600
    71728
    81056
    90528
    100560
    109840

After:

    14752
    14816
    15584
    15584
    15664
    15664
    15664
    15664
    15664
    15664

Extracted from PR #8932.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>

Many tests start by exhausting all shapes, which is a slow process.
By exposing a method to directly move the bump allocator forward
we cut test runtime in half.

Before:
```
Finished tests in 1.544756s
```

After:
```
Finished tests in 0.759733s,
```