Enumerate all secret stack buffers

[hanno-becker]

FIPS compliance requires reliably zeroing any secret intermediate values. There is no bullet-proof way to do this in C, but one can at least explicitly zeroize secret buffers allocated on the stack.

This issue is to list all the secret stack buffers that would need zeroing.

[hanno-becker]

• polyvec_basemul_acc_montgomery_cached (poly_k.c) uses a secret poly_t accumulator on the stack. This should be rewritten to directly accumulate in the target polynomial.
• polyvec_basemul_acc_montgomery allocates a temporary mulcache on the stack. This is only used in indcpa_dec and the stack allocation should be inlined there, alongside the other stack buffers.
• Buffers in all poly_getnoise functions (poly_k.c)

[hanno-becker]

Stack buffer in polyvec_basemul_acc_montgomery_cached is removed in #727