[ruby/fiddle] Add `sym_defined?` methods to test if a symbol is defined (https://github.com/ruby/fiddle/pull/108)

I would like to check if a symbol is defined before trying to access it.
Some symbols aren't available on all platforms, so instead of raising an
exception, I want to check if it's defined first.

Today we have to do:

```ruby
begin
  addr = Fiddle::Handle.sym("something")
  # do something
rescue Fiddle::DLError
end
```

I want to write this:

```ruby
if Fiddle::Handle.sym_defined?("something")
  addr = Fiddle::Handle.sym("something")
  # do something
end
```

https://github.com/ruby/fiddle/commit/9d3371de13

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

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