Previously Time.now was switched to use Time.new as it added support for
the in: argument. Unfortunately because Class#new is a cfunc this
requires always allocating a Hash.

This commit switches Time.now back to using a builtin time_s_now. This
avoids the extra Hash allocation and is about 3x faster.

    $ benchmark-driver -e './ruby;3.1::~/.rubies/ruby-3.1.0/bin/ruby;3.0::~/.rubies/ruby-3.0.2/bin/ruby' benchmark/time_now.yml
    Warming up --------------------------------------
                  Time.now     6.704M i/s -      6.710M times in 1.000814s (149.16ns/i, 328clocks/i)
    Time.now(in: "+09:00")     2.003M i/s -      2.112M times in 1.054330s (499.31ns/i)
    Calculating -------------------------------------
                               ./ruby         3.1         3.0
                  Time.now     7.693M      2.763M      6.394M i/s -     20.113M times in 2.614428s 7.278710s 3.145572s
    Time.now(in: "+09:00")     2.030M      1.260M      1.617M i/s -      6.008M times in 2.960132s 4.769378s 3.716537s

    Comparison:
                               Time.now
                    ./ruby:   7693129.7 i/s
                       3.0:   6394109.2 i/s - 1.20x  slower
                       3.1:   2763282.5 i/s - 2.78x  slower

                 Time.now(in: "+09:00")
                    ./ruby:   2029757.4 i/s
                       3.0:   1616652.3 i/s - 1.26x  slower
                       3.1:   1259776.2 i/s - 1.61x  slower