Bytecode + block decomposition: temporary terms were not correctly output

Temporary terms computed in previous blocks were not used in the bytecode
output of a given block. This was inefficient (because this means that
expressions already computed and store in the temporary terms vector would be
recomputed), and incidentally it would break the external functions
output (because it would trigger a lookup in the “TEF terms”, which would thus
fail).

Closes: #115