Path to FastLDF

[penelopeysm]

FastLDF works with most things and #1113 includes tests to make sure that it behaves the same way as LogDensityFunction, which is reassuring. But there are still some outstanding things.

Done	TL;DR	Issue	PR
✅	Make InitContext fast too	-	#1125
✅	Construct ParamsWithStats from vector + FastLDF	#1119	#1129
✅	Provide a way to opt out of TSVI	#1086	#1128
✅	Figure out why FastLDF is slow with untyped VarInfo	#1113 (comment)	Done in #1113 (86d8a73)
✅	Use Fast InitContext for logjoint, returned, predict, etc.	-	#1130
✅	Allocation tests	#1126	Done in #1113 (4b324b0)
✅	Correctness tests for thread-safe evaluation	-	#1132 (6467279)
✅	Maybe unify FastEvalVectorContext and InitFromParams. This is doable but a bit of a faff because the former handles both linked and unlinked values, whereas the latter only handles unlinked values (we also need to figure out logjac for all four possibilities). Personally, I think this is not mandatory for v0.39 as long as we keep FastEvalVectorContext internal.	-	#1132

[penelopeysm]

Every item on the whole checklist has been done somewhere, so I'll close this. Please refer to the links for more info.