toStringHex and toStringBinary strip off leading zeros

[soc]

They weren't stripped off originally, but that was changed in 78d0f06#diff-2e8fd6fa1728464bf16d731d758169acdbb204b74b6df173d91992d442e60524.

I did the original approach intentionally, because I felt that stripping of zeroes made it hard to visually compare different numbers in that format.

Now it turns out that it's not only a convenience, but that stripping off zeroes leads to bugs and other languages have added special lints for such APIs, see https://rules.sonarsource.com/java/RSPEC-4425.

I'd propose changing back to the previous behavior.

[soc]

@dinfuehr any objections against changing it back?