lcm method overflows for large inputs

[johnw42]

The lcm method can easily overflow when product of the inputs is too big to fit in the type of the arguments. I suggest adding a checked variant of lcm and related methods that uses checked_mul in the final step.

[johnw42]

I found this issue while working on a type that uses machine integers when possible and automatically overflows into a BigInt when necessary. I was surprised to see a panic in my unit tests. It was easy enough to work around by using checked_mul on the arguments to ensure lcm will succed, but the lcm method seems like a footgun for the unwary, and a checked variant would succeed in more cases than my workaround does.