Functions as first-class objects

[Akuli]

• [x] methods as first-class objects
• [x] port to self-hosted
• [x] figure out if this can already be used in self-hosted

Stuff to do in separate pull requests (checked = issue made):

• [x] implement something like atexit() #250
• [x] get rid of #define meth_... when it causes troubles with first-class methods #252
• [x] type syntax for "function that takes arguments of these types and returns this type" #249
  • [x] add test about two meanings of foo.bar(), can be an attribute being called or a method #251
• [x] .partial() method #254
• [x] sed -i 's/\bclass_/type_/g' $(git ls-files) because class_Foo is no longer necessarily a class, can also represent a function type #253