quicvarint: export Min and Max constants

[ydnar]

This is a cherry-pick from a WIP branch that adds two exported constants: quicvarint.Min and quicvarint.Max.

[codecov]

Codecov Report

Merging #3253 (78c2c75) into master (8ecbb05) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3253   +/-   ##
=======================================
  Coverage   85.48%   85.48%           
=======================================
  Files         132      132           
  Lines        9799     9799           
=======================================
  Hits         8376     8376           
  Misses       1048     1048           
  Partials      375      375           

Impacted Files	Coverage Δ
quicvarint/varint.go	78.38% <ø> (ø)

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