std.bigint: support construction from Int128

[Prashant-900]

This PR adds explicit BigInt constructors for Int128 resolving issue #10945.

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Testing this PR locally

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[ibuclaw]

You'd likely need an opAssign too, so it's not only for construction.

[Prashant-900]

Added assignment from Int128 and unit tests for a sanity check. While doing so, I noticed an inconsistency:

Int128 x = 10L;

This works, but:

Int128 x;
x = 10L; // error

does not. Assigning an int to Int128 also fails. Is this expected behavior, or should this be raised as an issue?

[0xEAB]

@ibuclaw Do you consider the added unittests to be sufficient?