Add getting strings from string constants

[rj45]

Here's the updated version of this change.

It turns out that other constants can also be turned into strings allowing you to get their binary value, so I am also using this to get the value of structs and arrays of words. There is probably a better way than converting it to a string (especially as utf8 might make things complicated) but I haven't looked more into it.

[aykevl]

FYI: there was a bug in this PR, see 9edb640.

[rj45]

Thanks for fixing that @aykevl, alas I don't have a windows box to test these things on. I thought the original type was unsigned long but size_t makes sense. I realize now that unsigned long would be 32 bit on windows and it needs to be 64 bit. I will use size_t in the future for lengths.

[aykevl]

I will use size_t in the future for lengths.

Always use the type as indicated in the original signature. In this case, the signature is as follows:

const char *LLVMGetAsString(LLVMValueRef C, size_t *Length);

It says size_t, so that's what you should use. If it said unsigned long, C.ulong would be the Go type. That way, the type is always the same, no matter how that particular type is defined on a given platform.