publicize QUIC varint reading and writing

[marten-seemann]

Packages outside of quic-go might want to make use of QUIC's varint format. We should expose the respective functions, so they don't have to reimplement this.

Specifically, MASQUE uses varints for the Stream Chunk encoding.

[codecov]

Codecov Report

Merging #2973 (f922688) into master (602212e) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2973      +/-   ##
==========================================
- Coverage   86.12%   86.03%   -0.09%     
==========================================
  Files         135      135              
  Lines        9379     9448      +69     
==========================================
+ Hits         8077     8128      +51     
- Misses        943      959      +16     
- Partials      359      361       +2     

Impacted Files	Coverage Δ
framer.go	100.00% <100.00%> (ø)
http3/client.go	88.61% <100.00%> (ø)
http3/frames.go	80.00% <100.00%> (ø)
http3/server.go	69.88% <100.00%> (+1.06%)	⬆️
internal/handshake/crypto_setup.go	66.59% <100.00%> (ø)
internal/handshake/session_ticket.go	100.00% <100.00%> (ø)
internal/wire/ack_frame.go	94.78% <100.00%> (ø)
internal/wire/connection_close_frame.go	94.74% <100.00%> (ø)
internal/wire/crypto_frame.go	95.45% <100.00%> (ø)
internal/wire/data_blocked_frame.go	100.00% <100.00%> (ø)
... and 17 more

---

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[lucas-clemente]

WDYT about s/VarInt//g in the function names in the package? quickvarint.ReadVarInt reads a bit redundant.

[marten-seemann]

That's what the second commit does.

[marten-seemann]

But it looks like I forgot WriteVarInt. Fixed now.