clang::Cursor::enum_val_unsigned should return Option<u64>

[fitzgen]

(Similar to #127)

Right now it always returns u64, but the FFI call it makes says

http://clang.llvm.org/doxygen/group__CINDEX__TYPES.html#gaf7cbd4f2d371dd93e8bc997c951a1aef

Retrieve the integer value of an enum constant declaration as an unsigned long long.

If the cursor does not reference an enum constant declaration, ULLONG_MAX is returned. Since this is also potentially a valid constant value, the kind of the cursor must be verified before calling this function.

We shouldn't force callers to check the kind of the cursor, we should do the check ourselves in this function and return an option. This would result in one less foot gun.

I can mentor anyone who'd like to pick up this bug.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[afluth]

I would like to take a swing at this.

[emilio]

Sure, thanks for it! :)

[jcdyer]

@afluth, I've got PR #220 coming in that (if accepted) reorganizes the calling code in enum_ty.rs a little bit.

[afluth]

@jcdyer, thanks for the heads up!