Changes related to creation of a modular backend in multiprecision [SyncWith: crypto3-multiprecision#70] #281

All 75 tests pass in 1m 2s

  28 files +  24   28 suites +24 1m 2s ⏱️ + 1m 2s
  75 tests +  70   75 ✔️ +  70 0 💤 ±0 0 ❌ ±0
160 runs +150 160 ✔️ +150 0 💤 ±0 0 ❌ ±0

Results for commit 28f1ca2. ± Comparison against earlier commit bad7aa5.