Various LogArray improvements

[spl]

• More documentation
• More tests
• Consistent naming
• More error checking
• Better error reporting
• Cleaned up and unified entry and decode functions
• Cleaned up future-based functions (removed Box/dyn, used combinators)
• Increased code coverage in tests
• Fixed monotonic check

[codecov-io]

Codecov Report

Merging #41 into master will decrease coverage by 0.12%.
The diff coverage is 74.37%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   77.16%   77.03%   -0.13%     
==========================================
  Files          21       21              
  Lines        7331     7478     +147     
  Branches      902      946      +44     
==========================================
+ Hits         5657     5761     +104     
- Misses        829      830       +1     
- Partials      845      887      +42

Impacted Files	Coverage Δ
src/structure/adjacencylist.rs	84.79% <100%> (ø)	⬆️
src/structure/wavelettree.rs	81.59% <100%> (ø)	⬆️
src/structure/logarray.rs	74.3% <74.19%> (-1.48%)	⬇️

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[spl]

It's interesting that Codecov shows that the coverage of logarray.rs has decreased. I worked a lot locally using grcov to increase it. I don't really understand Codecov's numbers, so I'm not sure what's happening.

[spl]

Just to add to that, here's what I see from the grcov HTML output for logarray.rs:

Reference	Line Coverage	Functions
master (665037d)	86.5% (307/355)	55.0% (94/171)
logarray-improvement (72aee3b)	90.2% (453/502)	56.6% (138/244)

[matko]

Wow, thanks a lot for this! You really made the code a lot easier to read.
Also, good job spotting that monotonic check bug.

I'm not entirely sure how the coverage is measured, but you definitely increased it, considering you added a whole bunch of tests. @rrooij any ideas?

I got no real improvements or review comments, so I'll just merge as is. Thanks again!