Add support for bitfields

[phronmophobic]

Bitfields in structs are ignored which can lead to crashes. The size of individual elements are too big and the struct class on the jvm side ends up much larger than the struct on the c side.

[phronmophobic]

In version 1.0, support was added for marking bitfields in easy-api and making sure bitfields don't break anything, but accessing fields of structs that have bitfields is not yet supported since it's not supported by jna.

java-native-access/jna#423