fix various errors and warnings seen on clang and gcc

…d-flag

Add CT_UNDER_CONSTRUCTION to indicate that a type is being mutated

(Kumar will fix this soon!)

Move CT_CUSTOM_FIELD_POS and CT_WITH_PACKED_CHANGE to ct_flags_mut

Replace ct_is_hidden with asserts

fix headers to avoid duplicated definitions

…dsafe

fix thread safety of `_realize_c_struct_or_union`

Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>