Fixed array sizes: a negative size becomes 0, and absurd sizes are accepted #170

struct S {
    a: u8[-3]                    // builds, as u8[0]
    b: u8[9223372036854775807]   // builds
}

TypeRef::from_grammar clamps a negative size with .max(0), so u8[-3] silently becomes u8[0]. Nothing caps a fixed size, so a field can declare more elements than any target can hold.

Fix: a negative size is an error, and so is an exact length of 0. Cap fixed sizes at u32::MAX, with each target's ceiling checked when generating. Both are part of the bounds design (core#98; issues 4 and 32 in docs' Proposal D).