Provide a Context method to construct ArrayType and FunctionType

[vaivaswatha]

inkwell::context::Context has methods to construct most other types, except vector, array and function types. Providing methods for these would be useful. Currently I see no other way of constructing an ArrayType, except match over every possible type for the element type, and call array_type on that. That's quite tedious.

[vaivaswatha]

I just realised that we can build ArrayType and FunctionType from a BasicTypeEnum, so this isn't that important. But it would be a nice to have. Please close the Issue if you think we don't need it.