fix: idiomatic math operations

[Stebalien]

• prefer is_positive/negative/zero over comparisons with TokenAmount::zero (saves an allocation).
• Prefer div_floor over div_rem

[codecov-commenter]

Codecov Report

Merging #591 (70d69a2) into master (b7ad2c5) will increase coverage by 0.12%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #591      +/-   ##
==========================================
+ Coverage   83.92%   84.04%   +0.12%     
==========================================
  Files          88       88              
  Lines       18155    18155              
==========================================
+ Hits        15236    15258      +22     
+ Misses       2919     2897      -22     

Impacted Files	Coverage Δ
actors/reward/src/state.rs	81.08% <0.00%> (ø)
actors/reward/src/lib.rs	82.92% <100.00%> (ø)
actors/cron/src/lib.rs	93.10% <0.00%> (-3.45%)	⬇️
actors/miner/src/lib.rs	80.53% <0.00%> (+0.05%)	⬆️
actors/verifreg/src/lib.rs	72.85% <0.00%> (+0.15%)	⬆️
test_vm/src/lib.rs	88.85% <0.00%> (+0.46%)	⬆️
runtime/src/util/message_accumulator.rs	94.54% <0.00%> (+0.90%)	⬆️
actors/miner/src/deadline_info.rs	96.62% <0.00%> (+16.85%)	⬆️