Domain-separate intrinsics

[adlerjohn]

Following up on #818 and #1119, we should:

• Domain-separate intrinsics, i.e. __size_of() and __is_reference_type(). #1531
• Forbid double leading underscores in function, variable, struct, enum, etc. names. #1533
• Include an intrinsics module in the standard library that simply wraps existing intrinsics. #1532

[otrho]

Do we prefer __size_of() to just using intrinsics::size_of() in user code?

[adlerjohn]

We would prefer users use intrinsics::size_of(), so that they never need to know that intrinsics even exist. Usage of intrinsics should be limited to the standard library (and then, only as directly wrapped).

[mohammadfawaz]

A lot of work went in recently to clean intrinsics up (both in the standard library and the compiler). I think this can be closed now.

Future intrinsics can now be added quite easily, as needed.