value_to_usize loses precision

[bnjbvr]

On 32-bits architectures, usize is 32-bits, by definition, so any conversion from a larger type into this will drop some bits. On 64-bits architecture, converting from 128-bits to 64-bits will also lose bits. It seems that it's only used for key paths computations, so for instance on 64-bits, that would require a static access in a list at an index >= 2**64 to cause an issue, so it's rather unlikely to happen, but it's still a real bug.

[davidpdrsn]

Probably worth testing if we can change that function to only handle usize and not cast the type integer types, like it does now. So only return Some for Value::usize.

I wrote it like this originally because I was worried about integer literals defaulting to i32 would be an issue but I haven't actually tested that.

[davidpdrsn]

We could also just panic if a lossless conversion fails. Like you say hitting that requires accessing very large lists.