Add ilog10 result range hints

[EFanZh]

This PR adds hints that the return value of T::ilog10 will never exceed T::MAX.ilog10().

This works because ilog10 is a monotonically nondecreasing function, the maximum return value is reached at the max input value.

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r? @Mark-Simulacrum

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[Noratrieb]

Is this motivated by concrete codegen improvements (the codegen tests are quite artificial)? assume/assert_unchecked always has the potential to go either way wrt performance, so it would be good to know whether you've confirmed that this improves real-world code

[jhorstmann]

Interesting that llvm can already infer some bounds, but apparently not the tightest ones. Looking at the following functions in compiler explorer, there are no panics:

pub fn test_u8(i: std::num::NonZeroU8, a: &[u32; 3]) -> u32 {
    a[i.ilog10() as usize]
}

pub fn test_u16(i: std::num::NonZeroU16, a: &[u32; 8]) -> u32 {
    a[i.ilog10() as usize]
}

pub fn test_u32(i: std::num::NonZeroU32, a: &[u32; 13]) -> u32 {
    a[i.ilog10() as usize]
}

pub fn test_u64(i: std::num::NonZeroU64, a: &[u32; 23]) -> u32 {
    a[i.ilog10() as usize]
}

These functions with tighter bounds would make for nicer codegen test.