std.math.big: require sufficient capacity for aliased params

[aaron-ang]

close #6167

We first check if the Managed instances are distinct but share the same limbs field in limbsAliasDistinct.

We then assert in ensureAliasAwareCapacity that an aliased Managed parameter has enough capacity such that it will not trigger a reallocation. This should prevent use-after-free.

Identical Managed instances should not result in use-after-free since toConst is called after any reallocation is done, so the underlying instances should point to the same limbs buffer during the actual arithmetic operation.

[aaron-ang]

hi @andrewrk could you trigger the CI?