encoding/binary: add AppendUvarint and AppendVarint

[thatradius]

#50601 introduced AppendByteOrder, so it would be nice if we had

func AppendUvarint(buf []byte, x uint64) []byte
func AppendVarint(buf []byte, x int64) []byte

[ianlancetaylor]

CC @dsnet

[rsc]

Seems OK and it would match not just binary.LittleEndian.AppendUint32 but also things like strconv.AppendQuote.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/400176 mentions this issue: encoding/binary: add AppendVarint AppendUvarint