Phase 0 #9. cgen struct fields loopendbuf/loopcontbuf/yieldbuf/
deferbuf change from `*str`/`**node` over-allocated arena chunks
to `[]str`/`[]*node` slices. The 4 alloc sites in cgeninit drop
the byte-count form (`LOOP_MAX*24u64`, `DEFER_MAX*8u64`) for
element-count (`LOOP_MAX: u64`, `DEFER_MAX: u64`). 10 caller
sites in cgenstmt.ww/cgenexpr.ww use `[i]` indexing which works
identically for slice-shaped struct fields.
Two-line let-then-assign idiom for the 4 inits is a real checker
limitation: alloc's element-deferred `[]u8` → `[]T` retype only
fires in let-init (checkletassign N_TSLICE LHS), and cglet's
alloc-slice writeback shortcut (cgenstmt.ww:577) only fires in
let-init too. Direct `c.field = alloc([], N)!` would silently
emit a scalar alloc with a junk slice header. Filed #49 for the
checker enhancement; the let-then-assign is Hare-idiomatic in
the meantime.
Verified 132/132 + 995_self_rebuild byte-identity.
Hojun-Cho
4972ab4a5c