Replace tempdir dependency with tempfile

[thanethomson]

Closes #851

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #883 (bfb8607) into master (b032801) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #883     +/-   ##
========================================
- Coverage    69.8%   69.7%   -0.1%     
========================================
  Files         200     200             
  Lines       16314   16322      +8     
========================================
- Hits        11393   11390      -3     
- Misses       4921    4932     +11     

Impacted Files	Coverage Δ
light-client/src/store/sled.rs	77.0% <100.0%> (ø)
light-client/src/store/sled/utils.rs	65.0% <100.0%> (ø)
tendermint/src/signature.rs	56.4% <0.0%> (-4.2%)	⬇️
light-client/src/types.rs	30.1% <0.0%> (-0.9%)	⬇️
testgen/src/header.rs	78.1% <0.0%> (-0.7%)	⬇️
testgen/src/vote.rs	83.7% <0.0%> (ø)
tendermint/src/node.rs	65.9% <0.0%> (+0.2%)	⬆️

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