Set types as attributes on the cstruct object

[Schamper]

Closes #113

Some micro benchmarks:

from dissect.cstruct import cstruct

cdef = """
#define X 512

enum MyEnum {
    A,
    B,
    C
};

struct test {
    uint32 a;
};
"""
cs = cstruct()
cs.load(cdef)

Before:

In: %timeit getattr(t.cs, "X")
58.1 ns ± 1.04 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)

In: %timeit getattr(t.cs, "MyEnum")
227 ns ± 0.912 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

In: %timeit getattr(t.cs, "test")
219 ns ± 1.47 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

After:

In: %timeit getattr(t.cs, "X")
31.2 ns ± 0.0331 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)

In: %timeit getattr(t.cs, "MyEnum")
33.1 ns ± 0.0884 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)

In: %timeit getattr(t.cs, "test")
31.3 ns ± 0.0571 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)