Lazy load type mappings

[andynu]

This appears to fix issue #2256. I believe it should have the same memory implications as the intention in #2199. The type maps are loaded once when they are first called. Only one instance of the mapping is ever created.

Fortunately this was enough to solve the problem, but if there were a need, this could easily be worked into the clear_cache! API to reset it at a later point in the application just by setting the @type_map to nil.

(clean patch against the master branch, nothing from the release70 here)