cryptobyte: AddUint*LengthPrefixed API perfomance optimization

[mateusz834]

benchmark old ns/op new ns/op delta
BenchmarkLengthPrefixed-4 1345 265 -80.27%

benchmark old allocs new allocs delta
BenchmarkLengthPrefixed-4 17 1 -94.12%

benchmark old bytes new bytes delta
BenchmarkLengthPrefixed-4 777 64 -91.76%

Fixes golang/go#54059

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[gopherbot]

This PR (HEAD: 5dbe5c7) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/428475 to see it.

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See the Wiki page for more info

[gopherbot]

This PR (HEAD: a633e5f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/428475 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info