Fix crash on generic methods

[apiology]

Fix crash on methods declared as independently generic. This doesn't yet support inferring types on methods which are generic based on their parameter, but at least avoids crashing when they are defined:

/Users/broz/src/solargraph/lib/solargraph/complex_type/unique_type.rb:71:in `resolve_generics': undefined method `generics' for an instance of Solargraph::Pin::Method (NoMethodError)

          idx = definitions.generics.index(subtypes.first&.name)
                           ^^^^^^^^^
	from /Users/broz/src/solargraph/lib/solargraph/complex_type.rb:118:in `block in resolve_generics'
	from /Users/broz/src/solargraph/lib/solargraph/complex_type.rb:118:in `map'
	from /Users/broz/src/solargraph/lib/solargraph/complex_type.rb:118:in `resolve_generics'
	from /Users/broz/src/solargraph/lib/solargraph/source/chain.rb:132:in `block in infer_first_defined'
	from /Users/broz/src/solargraph/lib/solargraph/source/chain.rb:123:in `each'
	from /Users/broz/src/solargraph/lib/solargraph/source/chain.rb:123:in `infer_first_defined'
        from /Users/broz/src/solargraph/lib/solargraph/source/chain.rb:85:in `infer'

[lekemula]

Nice catch! Are there any methods in Ruby's Standard Library (rbs signatures) that use standalone generic methods?

[apiology]

Easy example:
array = [1,2,3] # type is Array
array.fetch(1, "foo") # type is Union<Integer, String>

A couple of others, also in Array - but other classes do this as well:

• The singleton method [] - the function that turns [1,2,3] into an Array object.
• Array#+, which returns Array<Union<A, B>>, if A and B are the types on the left and right hand side

[apiology]

I was trying to get this to do something useful with type checking, but it's probably worth populating the generics variable now from RBS in case someone beats me to do it - will push up that commit in a moment

[castwide]

This definitely looks like a move in the right direction. I'm happy to merge it now and continue testing it locally. Thanks, @apiology!