Implement eltype for list

[saolof]

...Otherwise this can be a very easy to miss source of type instability. Adding this line gave me a fivefold speedup on a problem I had

[codecov-commenter]

Codecov Report

Merging #68 (1bea603) into master (2dd9b36) will decrease coverage by 0.80%. The diff coverage is 0.00%.

:exclamation: Current head 1bea603 differs from pull request most recent head fb3bb25. Consider uploading reports for the commit fb3bb25 to get more accurate results

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   77.30%   76.50%   -0.81%     
==========================================
  Files           7        7              
  Lines         379      383       +4     
==========================================
  Hits          293      293              
- Misses         86       90       +4     

Impacted Files	Coverage Δ
src/PersistentList.jl	64.00% <0.00%> (-5.57%)	:arrow_down:

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[saolof]

I'm seeing that #39 tried making a PR to do this several years ago, but that the build would fail and the PR didn't get merged. This does in fact implement the same functionality but the build seems to pass.

I'll try adding the full set of Tests from that PR with the names fixed so that they work