Add typing for ImageSectionHeader::characteristics

[jan-auer]

SectionCharacteristics is implemented as a public newtype around the raw
u32, giving access to the inner value in case flags are not implemented. The
wrapper follows the same pattern used for flags in the TPI, which parses the
flag values lazily.

There is special handling for the mutually exclusive alignment values, as well
as the _NO_PAD flag. Otherwise, all flags are exposed directly as booleans.

Fixes #106

[jan-auer]

The constants are intentionally not exposed yet, that will be dealt with at once for #120.

cc @mstange, in case you'd like changes before this goes in

[mstange]

Looks great to me, thank you!