Add ConstantAggregateZero ConstantExpr and a few more methods

[vihanb]

So over the past year I've been working on adding various features to llvm-node for the language I've been working on. I first made a PR with #42 but I've put all my changes in one PR to be more organized and to avoid merge issues.

Anyway, so another thing is looks like both me and @ovr happened to add an IntegerType class but I've worked out the conflicts with those and all tests (for my language using llvm-node) pass.

[MichaReiser]

Hi @vihanb

Thank you for fixing the review comments. However, the build is still failing. Can you take a look? I would prefer merging this soon to avoid that it gets any bigger.

[vihanb]

looks like still errors with IntegerTypes... will take a closer look at when I have more time in a few weeks

[vihanb]

So looks like I have .getBitWidth() as a getter integerTyObject.bitWidth on my branch. I'm thinking it might be more idiomatic to have as getter but I'll change back for compatibility in existing projects