Remove whole-struct assignment in `polyvec` functions

[mkannwischer]

For the CBMC proof of polyveck_decompose added in #92, some workarounds were needed due to limitations in CBMC:

This issue is to track diffblue/cbmc#8617.
Further functions were identified in #151.

Once that is resolved and CBMC is updated, we should be able to remove the workarounds in all affected functions

• polyveck_decompose
• polyvecl_add
• polyveck_add
• polyveck_sub
• polyveck_use_hint
• polyveck_reduce
• polyvecl_reduce
• polyvecl_ntt
• polyveck_ntt
• polyveck_addq
• polyveck_power2round
• polyvecl_invntt_tomont
• polyveck_invntt_tomont
• polyveck_uniform_eta
• polyvecl_uniform_eta
• polyvec_matrix_expand