Function objects, @|| meta key

[irh]

It would be useful to be able to define function objects in Koto.

This could be achieved by adding a @|| meta key that can be defined in Maps, which would then allow the map to be treated as a function.

e.g.

my_function = 
  @||: || 42

my_function_with_args =      
  @||: |x, y| x + y

my_instance_function =      
  data: [1, 2, 3]
  @||: |self| self.data

print my_function()
# 42

print my_function_with_args 2, 3
# 5

print my_instance_function()
# [1, 2, 3]

[irh]

Implemented in #197.