Add fltUser and fltUserStructures

[mvforell]

This PR adds the definitions from um/fltUser.h and shared/fltUserStructures.h.

All tests pass and I'm able to compile a binary using parts of the exposed interface, so I'm pretty sure I haven't done anything horribly wrong. However I'm not sure what the _Struct_size_bytes_(...) annotations in the headers mean, for example:

// shared/fltUserStructures.h, line 383
typedef _Struct_size_bytes_(sizeof(INSTANCE_BASIC_INFORMATION) * InstanceNameLength) struct _INSTANCE_BASIC_INFORMATION {
    ULONG NextEntryOffset;
    USHORT InstanceNameLength;
    USHORT InstanceNameBufferOffset;
} INSTANCE_BASIC_INFORMATION, *PINSTANCE_BASIC_INFORMATION;

Are they relevant to this crate?

[roblabla]

Duplicate of #923.

[mvforell]

Whoops, didn't see that. Sorry.