Split up operator""_method

[beached]

When wrapping calls to crow from other code bases, I have had to call operator""_method explicitly. Breaking it into two methods, method_from_string and operator""_method allows one to compose this code elsewhere.

[The-EDev]

Everything seems to be in order. Thank you for your work.

[beached]

Thanks, this saves my code from saying operator""_method( str.data( ), str.size( ) ) explicitly