To avoid stack overflow, Ruby splits compilation of large arrays
into smaller arrays, and concatenates the small arrays together.
It previously used newarray/concatarray for this, which is
inefficient.  This switches the compilation to use pushtoarray,
which is much faster. This makes almost all literal arrays only
allocate a single array.

For cases where there is a large amount of static values in the
array, Ruby will statically compile subarrays, and previously
added them using concatarray.  This switches to concattoarray,
avoiding an array allocation for the append.

Keyword splats are also supported in arrays, and ignored if the
keyword splat is empty.  Previously, this used newarraykwsplat and
concatarray.  This still uses newarraykwsplat, but switches to
concattoarray to save an allocation.  So large arrays with keyword
splats can allocate 2 arrays instead of 1.

Previously, for the following array sizes (assuming local variable
access for each element), Ruby allocated the following number of
arrays:

  1000 elements: 7 arrays
 10000 elements: 79 arrays
100000 elements: 781 arrays

With these changes, only a single array is allocated (or 2 for a
large array with a keyword splat.

Results using the included benchmark:

```
                       array_1000
            miniruby:     34770.0 i/s
   ./miniruby-before:     10511.7 i/s - 3.31x  slower

                      array_10000
            miniruby:      4938.8 i/s
   ./miniruby-before:       483.8 i/s - 10.21x  slower

                     array_100000
            miniruby:       727.2 i/s
   ./miniruby-before:         4.1 i/s - 176.98x  slower
```

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>