Import TruffleRuby implementation

Fix GH-145

lib/fiddle/truffleruby.rb is based on
https://github.com/oracle/truffleruby/blob/master/lib/truffle/truffle/fiddle_backend.rb
.

Here are changes for it:

* Add `Fiddle::Types::VARIADIC`
* Add `Fiddle::Types::CONST_STRING`
* Add `Fiddle::Types::BOOL`
* Add `Fiddle::ALIGN_BOOL`
* Add `Fiddle::SIZEOF_BOOL`
* Add `Fiddle::SIZEOF_CONST_STRING`
* Add support for specifying type as not only `Fiddle::Types::*` but also
  `Symbol` like `:int`
* Add `Fiddle::Error` as base the error class
* Add support for `Fiddle::Pointer.malloc {}`
  `Fiddle::Pointer`
* Add support for `Fiddle.free(#to_int)`
* Accept `Fiddle::Function(need_gvl:)` but it's just ignored
* `Fiddle::Function#initialize`: Add an argument validation
* `Fiddle::Function#initialize`: Keep arguments as instance variables for
  getters
* Add support for `Fiddle::Handle.sym`
* Add support for `Fiddle::Handle.[]`
* Add support for `Fiddle::Handle.sym_defined?`
* Add support for `Fiddle::Handle#sym`
* Add support for `Fiddle::Handle#[]`
* Add support for `Fiddle::Handle#sym_defined?`
* Add support for `Fiddle::Pointer.malloc`
* Add support for `Fiddle::Pointer.to_ptr(#to_ptr)`
* Add support for `Fiddle::Pointer#free=`
* Add `Fiddle::Pointer#freed?`
* Add support for `Fiddle::Pointer#call_free`
* Add support for `Fiddle::Pointer#to_i`
* Add support for `Fiddle::Pointer#to_int`
* Add support for `Fiddle::Pointer#ptr`
* Add support for `Fiddle::Pointer#+@`
* Add support for `Fiddle::Pointer#ref`
* Add support for `Fiddle::Pointer#-@`
* Add support for `Fiddle::Pointer#null?`
* Add support for `Fiddle::Pointer#to_s`
* Add support for `Fiddle::Pointer#to_str`
* Add support for `Fiddle::Pointer#inspect`
* Add support for `Fiddle::Pointer#<=>`
* Add support for `Fiddle::Pointer#==`
* Add support for `Fiddle::Pointer#eql?`
* Add support for `Fiddle::Pointer#+`
* Add support for `Fiddle::Pointer#-`
* Add support for `Fiddle::Pointer#[]=`
* Add support for `Fiddle::Pointer#size`
* Add support for `Fiddle::Pointer#size=`
* Add `Fiddle::ClearedReferenceError`
* Add no-op `Fiddle::Pinned`
* Add `Fiddle::NULL`

Some features are still "not implemented". So there are some "omit"s
for TruffleRuby in tests.

.github/workflows/ci.yml

@@ -26,13 +26,15 @@ jobs:
           - '3.2'
           - debug
           - jruby
+          - truffleruby
         include:
           - { os: windows-latest , ruby: mingw }
           - { os: windows-latest , ruby: mswin }
         exclude:
           - { os: macos-14  , ruby: '2.5' }
           - { os: windows-latest , ruby: '3.0' }
           - { os: windows-latest , ruby: debug }
+          - { os: windows-latest , ruby: truffleruby }
 
     steps:
       - uses: actions/checkout@v4

ext/fiddle/extconf.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 require 'mkmf'
 
-if RUBY_ENGINE == "jruby"
+unless RUBY_ENGINE == "ruby"
   File.write('Makefile', dummy_makefile("").join)
   return
 end

fiddle.gemspec

@@ -44,6 +44,7 @@ Gem::Specification.new do |spec|
     "lib/fiddle/pack.rb",
     "lib/fiddle/ruby.rb",
     "lib/fiddle/struct.rb",
+    "lib/fiddle/truffleruby.rb",
     "lib/fiddle/types.rb",
     "lib/fiddle/value.rb",
     "lib/fiddle/version.rb",