Prevent ensure(...) calls missing chained assertions

[JosiahWitt]

Closes #11

Breaking changes:

• Requires Go 1.14+
• Removes Fail() method, and replaces it with Failf(<msg>)
• Everything now uses Fatalf instead of Errorf, so tests fail immediately if something goes wrong

• Fails tests if ensure(...) is called and is missing chained assertions. For example:

  ensure(true) // Fails

[codecov-io]

Codecov Report

Merging #16 (f80ae47) into master (bb2d171) will decrease coverage by 0.42%. The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   98.62%   98.19%   -0.43%     
==========================================
  Files           5        5              
  Lines         145      166      +21     
==========================================
+ Hits          143      163      +20     
- Misses          1        2       +1     
  Partials        1        1              

Impacted Files	Coverage Δ
ensure.go	100.00% <ø> (ø)
ensurepkg/ensurepkg.go	88.00% <88.23%> (+1.33%)	:arrow_up:
ensurepkg/chain.go	100.00% <100.00%> (ø)
ensurepkg/run.go	100.00% <100.00%> (ø)
ensurepkg/run_table.go	100.00% <100.00%> (ø)

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