Callback fixes #40 and #41

[ahueck]

• Allow direct calls to TypeART callback functions, closes Directly calling callback functions causes a crash #40
• Fix type mismatch of pass vs. runtime w.r.t. stack alloca counter: i32 vs. i64 (i32 is used now), closes LLVM Type for leave_scope callback out of sync with CallbackInterface.h #41

[coveralls]

Pull Request Test Coverage Report for Build 451312762

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 86.269%

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Totals
Change from base Build 449198362:	0.03%
Covered Lines:	2111
Relevant Lines:	2447

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💛 - Coveralls