Use rb_str_buf_new() to improve JSON.parse() performance with short string

[Watson1978]

rb_str_buf_new always performs allocation twice (at str_alloc and ALLOC_N).
And the method does not allow embedded string even if the string is short enough.

VALUE
rb_str_buf_new(long capa)
{
    VALUE str = str_alloc(rb_cString);

    if (capa < STR_BUF_MIN_SIZE) {
        capa = STR_BUF_MIN_SIZE;
    }
    FL_SET(str, STR_NOEMBED);
    RSTRING(str)->as.heap.aux.capa = capa;
    RSTRING(str)->as.heap.ptr = ALLOC_N(char, (size_t)capa + 1);
    RSTRING(str)->as.heap.ptr[0] = '\0';

    return str;
}

Therefore, this PR uses rb_str_new instead to reduce the allocation with a short string.
This PR will improve the performance by about 16%.

Environment

• MacBook Pro (16-inch, 2019)
• macOS 10.15.5
• Intel Core i9 2.4 GHz
• Ruby 2.7.2

Before

Warming up --------------------------------------
   short_string_json    10.660k i/100ms
    long_string_json    10.058k i/100ms
Calculating -------------------------------------
   short_string_json    106.992k (± 1.4%) i/s -    543.660k in   5.082355s
    long_string_json    102.273k (± 1.0%) i/s -    512.958k in   5.016081s

After

Warming up --------------------------------------
   short_string_json    12.413k i/100ms
    long_string_json    10.457k i/100ms
Calculating -------------------------------------
   short_string_json    124.316k (± 0.8%) i/s -    633.063k in   5.092705s
    long_string_json    105.665k (± 0.9%) i/s -    533.307k in   5.047540s

Test code

require 'benchmark/ips'
require 'json'

short_string_json = {
  "a" => "b" * 23,
  "a" * 23 => "b"
}.to_json.freeze

long_string_json = {
  "a" => "b" * 50,
  "a" * 50 => "b"
}.to_json.freeze

Benchmark.ips do |x|
  x.report("short_string_json") { JSON.parse(short_string_json) }
  x.report("long_string_json") { JSON.parse(long_string_json) }
end

[Watson1978]

The change with the same effect was made at ruby/ruby@02c32b2

However, this PR is still useful for Ruby 2.7.x or below.

[Watson1978]

Seems this PR isn't necessary any more by #451