Pretty print function definition

[sharkdp]

This adds a new statement to the Insect language that allows
users to pretty-print a function definition by simply typing
the name of the function:

>>> f(x, y) = cos(x) + y^2

  f(x, y) = cos(x) + y^2

>>> f

  f(x, y) = cos(x) + y^2

>>> cos

  cos(x) = builtin function

>>> atan2

  atan2(x, y) = builtin function

>>> mean

  mean(x1, x2, …) = builtin function

closes #251