Fix issue #752

[tjanc]

Fixed

• primitive type names are now added to type registry
• release build: unresolved references are ignored in backend
• debug build: unresolved references are asserted on in backend

Not fixed

• snowcrash not recognizing nested sections in a primitive mixin.

[kylef]

Can you add a changelog entry too? Just so the next release is sure to mention the change and it doesn't slip by, something like:

## TBD

### Bug Fixes

* Prevents a potential segmentaiton fault crash during XYZ circumstances (possibly use an example in API Blueprint below).