run gofumpt, add a few more linters

[marten-seemann]

No functional changes expected.

[codecov-io]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #21   +/-   ##
=======================================
  Coverage   90.82%   90.82%           
=======================================
  Files           5        5           
  Lines         207      207           
=======================================
  Hits          188      188           
  Misses         10       10           
  Partials        9        9           

Impacted Files	Coverage Δ
encoder.go	100.00% <ø> (ø)
static_table.go	0.00% <ø> (ø)

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