A gemified version of the Symbol[str] method proposed in https://bugs.ruby-lang.org/issues/7854
call-seq:
Symbol.find str => Symbol or nil
Symbol[str] => Symbol or nil
str.interned? => Symbol or nil
str.to_existing_sym => Symbol or ArgumentError
Returns an extant symbol, which is the .to_sym of str, or
nil if no such symbol exists.
Symbol.find 'Object' #=> :Object
Symbol.find 'none such' #=> nil
Symbol['Object'] #=> :Object
Symbol['none such'] #=> nil
'Object'.interned? #=> :Object
'none such'.interned? #=> nil
'Object'.to_existing_sym #=> :Object
'none such'.to_existing_sym #=> (raises ArgumentError)