Move more NilClass methods to ruby

```
$ make benchmark ITEM=nilclass COMPARE_RUBY="/opt/rubies/ruby-master/bin/ruby"
/opt/rubies/3.4.2/bin/ruby --disable=gems -rrubygems -I../benchmark/lib ../benchmark/benchmark-driver/exe/benchmark-driver \
                    --executables="compare-ruby::/opt/rubies/ruby-master/bin/ruby -I.ext/common --disable-gem" \
                    --executables="built-ruby::./miniruby -I../lib -I. -I.ext/common  ../tool/runruby.rb --extout=.ext  -- --disable-gems --disable-gem" \
                    --output=markdown --output-compare -v $(find ../benchmark -maxdepth 1 -name 'nilclass' -o -name '*nilclass*.yml' -o -name '*nilclass*.rb' | sort)
compare-ruby: ruby 3.5.0dev (2025-06-02T13:52:25Z master cbd49ecbbe) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-06-02T22:47:21Z hm-ruby-nilclass 3e7f1f0466) +PRISM [arm64-darwin24]

|             |compare-ruby|built-ruby|
|:------------|-----------:|---------:|
|rationalize  |     24.056M|   53.908M|
|             |           -|     2.24x|
|to_c         |     23.652M|   82.781M|
|             |           -|     3.50x|
|to_i         |     89.526M|   84.388M|
|             |       1.06x|         -|
|to_f         |     84.746M|   96.899M|
|             |           -|     1.14x|
|to_r         |     25.107M|   83.472M|
|             |           -|     3.32x|
|splat        |     42.772M|   42.717M|
|             |       1.00x|         -|
```

This makes them much faster

benchmark/nilclass.yml

@@ -1,10 +1,16 @@
 prelude: |
   def a = nil
 benchmark:
+  rationalize:
+    nil.rationalize
+  to_c: |
+    nil.to_c
   to_i: |
     nil.to_i
   to_f: |
     nil.to_f
+  to_r: |
+    nil.to_r
   splat: |
     a(*nil)
 loop_count: 100000