Implement From<char> for u64

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@Ixrec

I thought From / Into were simply for infallible conversions where there's only one possible (or one obvious default) way of doing the conversion, such that no one could reasonably need to ask why this char value gets turned into that u32 value instead of some other u32 value.

Similar to the reasoning in Add non-`unsafe` `get_mut` for `UnsafeCell`, I'd suggest that the absence of the infallible conversion implies that there isn't a correct one. The error message could reasonably point us to use try_into.

I'm also sympathetic to the viewpoint that this particular conversion should be spelled something like

let n : u64 = 'a'.encode_utf32().into();

but I don't know if that's workable into a consistent API without deprecating some From impls.