more tests and fix next_back for iter_mut

[codecov[bot]]

Codecov Report

Merging #74 (9fafee2) into unsafe_iter_mut (fb002d4) will increase coverage by 13.48%. The diff coverage is 100.00%.

@@                 Coverage Diff                  @@
##           unsafe_iter_mut      #74       +/-   ##
====================================================
+ Coverage            82.26%   95.74%   +13.48%     
====================================================
  Files                    5        4        -1     
  Lines                  265      235       -30     
====================================================
+ Hits                   218      225        +7     
+ Misses                  47       10       -37     

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/ringbuffer_trait.rs	100.00% <100.00%> (+6.48%)	:arrow_up:
benches/bench.rs

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