3d: weaken allow_reading for entrypoints

[tahina-pro]

Consider the following 3D data format description:

entrypoint typedef struct _test {
  UINT16 x;
} test;

From this definition, currently, 3D generates F* code that does not typecheck, because entrypoints are not supposed to have a reader, and the way we currently enforce this requirement is to forbid such definitions.

This PR relaxes this requirement to allow those definitions, by casting entrypoint validators with validate_drop to weaken their types.