RFC: try to simplify information flow around inference

[JeffBezanson]

Attempting some cleanup here. cc @vtjnash

• Remove code for copying data from one LambdaInfo to another; instead have only one LambdaInfo.
• typeinf_edge was calling jl_specializations_insert on a LambdaInfo even if it already came from the cache.
• Combine creating and caching specializations into one step.
• Be more careful about where linfo->inInference is set to 1.
• In general try to avoid potential re-entrance problems in typeinf.

[vtjnash]

nice! I didn't realize this was so close to be being fully functional.

lgtm.

[StefanKarpinski]

[JeffBezanson]

Unfortunately had to give up some of the code reduction in order to handle deleted IR. Oh well.