wire: refactor header parsing to use quicvarint.Parse

[marten-seemann]

Part of #3663.

Now that we have a quicvarint.Parse (#4475), we can use that when parsing Long Header packets:

name                           old time/op    new time/op    delta
ParseInitial/without_token-16     114ns ± 4%      43ns ± 2%  -62.60%  (p=0.000 n=10+10)
ParseInitial/with_token-16        121ns ± 2%      54ns ± 1%  -55.22%  (p=0.000 n=10+8)
ParseRetry-16                     120ns ± 2%      53ns ± 2%  -55.76%  (p=0.000 n=10+10)

name                           old alloc/op   new alloc/op   delta
ParseInitial/without_token-16      192B ± 0%       96B ± 0%  -50.00%  (p=0.000 n=10+10)
ParseInitial/with_token-16         224B ± 0%      128B ± 0%  -42.86%  (p=0.000 n=10+10)
ParseRetry-16                      240B ± 0%      144B ± 0%  -40.00%  (p=0.000 n=10+10)

name                           old allocs/op  new allocs/op  delta
ParseInitial/without_token-16      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.000 n=10+10)
ParseInitial/with_token-16         5.00 ± 0%      2.00 ± 0%  -60.00%  (p=0.000 n=10+10)
ParseRetry-16                      5.00 ± 0%      2.00 ± 0%  -60.00%  (p=0.000 n=10+10)

[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 85.29%. Comparing base (3122ca0) to head (2384a76).
Report is 5 commits behind head on master.

Files	Patch %	Lines
internal/wire/header.go	82.61%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4481      +/-   ##
==========================================
+ Coverage   84.92%   85.29%   +0.37%     
==========================================
  Files         154      154              
  Lines       14676    14833     +157     
==========================================
+ Hits        12463    12651     +188     
+ Misses       1700     1679      -21     
+ Partials      513      503      -10     

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