Import TruffleRuby implementation

[kou]

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.

[kou]

This is based on GH-147. So we should not merge this before GH-147.

[eregon]

I changed the target branch so this PR shows the relevant diff.