Better error info in process() method checking.
Comment clarifying some code.
Refactored EntityRange static ifs.
Minor cleanup.
Style/cleanup.
DefaultDescriptor only needs Loader to get source, not ComponentTypeManager.
A method to get an exact-sized slice of a paged buffer.
Multiple changes, see details.

* Refactored most `PrototypeManager` code into a `MallocResourceManager`
that can serve as a base for user-defined resource managers.
* Source API is now much more explicit.
Added `isSequence()` and `isMapping()` methods to determine Source
type. Accessors such as `getSequenceValue()`/`getMappingValue()` now
must assert that source type matches.