3d: more meaningful error messages for misuses of [:consume-all]

[tahina-pro]

Reported by @Smfakhoury

The following 3D specification:

typedef struct _s {
UINT8 a[:consume-all];
UINT8 b[:consume-all];
} s;

rightfully fails because [:consume-all] removes the strong prefix property of its parser. But right now, EverParse fails with F* Error 19, i.e. a Z3 failure to prove that field a above with [:consume-all] has the strong prefix property. While expected, this is not palatable to the user.

In fact, as designed in #116, [:consume-all] is meant to be used only as the last field of a struct. Thus, I believe this should be checked syntactically (or, at worst, at the level of the 3D AST) and EverParse should reject uses of [:consume-all] for struct fields other than the last one, with a suitable error message.