builtin: rename type to "builtin_function_or_method" (#30)

* builtin: rename type to "builtin_function_or_method"

Also:
- remove all uses of "builtin" as a noun.
  Now it's always "built-in function" (or method).

Change-Id: I2c81bb19800c40ed0f781b77449baa63333cb5e7

* set: in x|y, require that y is a set if x is a set (#29)

The old functionality can still be accessed using x.union(y).

* type: clarify difference from Java

Change-Id: If36d4c5e97bb48c088356a7b538d4db0c165054c

* builtin: rename type to "builtin_function_or_method"

Also:
- remove all uses of "builtin" as a noun.
  Now it's always "built-in function" (or method).