Fix GH-145

Rename `lib/fiddle/jruby.rb` to `lib/fiddle/ffi_backend.rb` as a generic
ffi gem API based implementation.
JRuby and TruffleRuby use `lib/fiddle/ffi_backend.rb`.

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Co-authored-by: Benoit Daloze <eregontp@gmail.com>

Fix GH-104

lib/fiddle/jruby.rb is based on

https://github.com/jruby/jruby/blob/master/lib/ruby/stdlib/fiddle/jruby.rb
.

Here are changes for it:

* Move `Fiddle::TYPE_*` to `Fiddle::Types::*`
* Add `Fiddle::Types::VARIADIC`
* Add `Fiddle::Types::CONST_STRING`
* Add `Fiddle::Types::BOOL`
* Add `Fiddle::Types::INT8_T`
* Add `Fiddle::Types::UINT8_T`
* Add `Fiddle::Types::INT16_T`
* Add `Fiddle::Types::UINT16_T`
* Add `Fiddle::Types::INT32_T`
* Add `Fiddle::Types::UINT32_T`
* Add `Fiddle::Types::INT64_T`
* Add `Fiddle::Types::UINT64_T`
* Add more `Fiddle::ALIGN_*` for the above new `Fiddle::Types::*`
* Add more `Fiddle::SIZEOF_*` for the above new `Fiddle::Types::*`
* Add support for specifying type as not only `Fiddle::Types::*` but
also `Symbol` like `:int`
* Add support for variable size arguments in `Fiddle::Function`
* Add `Fiddle::Closure#free`
* Add `Fiddle::Closure#freed?`
* Add `Fiddle::Error` as base the error class
* Add `Fiddle::Pointer#call_free` and stop using `FFI::AutoPointer` in
`Fiddle::Pointer`
* Add support for `Fiddle::Pointer.malloc {}` `Fiddle::Pointer`
* Add support for `Fiddle::Pointer#free=`
* Add `Fiddle::Pointer#freed?`
* Use binary string not C string for `Fiddle::Pointer#[]`
* Add `Fiddle::Handle.sym_defined?`
* Add `Fiddle::Handle#sym_defined?`
* Add `Fiddle::Handle::DEFAULT`
* Add `Fiddle::ClearedReferenceError`
* Add no-op `Fiddle::Pinned`

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

(https://github.com/ruby/fiddle/pull/143)

Pick
https://github.com/ruby/ruby/commit/92865d8760e22dc5035a67e636fab3fbd7a77a26
from `ruby/ruby` repo.

---------

https://github.com/ruby/fiddle/commit/aad5a3bc79

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

(https://github.com/ruby/fiddle/pull/138)

See https://guides.rubygems.org/specification-reference/#metadata for changelog_uri metadata.

https://github.com/ruby/fiddle/commit/0bd8e96adc

https://bugs.ruby-lang.org/issues/18571

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>

https://github.com/ruby/fiddle/commit/e08c4c635e

Co-authored-by: Sutou Kouhei <kou@clear-code.com>

Drop supports for old versions, keeping 2.5 as CI supports it for
now.

https://github.com/ruby/fiddle/commit/90634e7c55

I don't use tool/sync_default_gem.rb because the last sync was incomplete.

Co-authored-by: Hiroshi SHIBATA <hsbt@ruby-lang.org>
Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>
Co-authored-by: sinisterchipmunk <sinisterchipmunk@gmail.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>

It's fixed in upstream.
https://github.com/MSP-Greg/ruby-loco/issues/4

https://github.com/ruby/fiddle/commit/2ae0ff4934

https://github.com/ruby/fiddle/commit/bb227c206d