coinage-layer task #88: 2-coin+1-entry and 1-coin+2-entry covers

find_two_coin_one_entry_cover and find_one_coin_two_entry_cover —
both with sharp None postconditions. Same accumulator pattern as the
multi-coin #87 series, with the dimensions split between coin and
entry Vecs. Each landed first-try clean.

Combined with the strengthened find_coin_entry_exact_cover (1c+1e
with sharp None) shipped earlier, we now have all coin/entry covers
of total size 2 and 3 with full sharp None.

267 verified, 0 errors.

Author: TorstenStueber