std.bigint: support construction from Int128

[Prashant-900]

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

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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?

[Bolpat]

Suggested change

	/// Construct `BigInt` from `Int128`
	import std.int128 : Int128;
	import std.int128 : Int128;
	/// Construct `BigInt` from `Int128`

Not sure if this is needed, but I think the doc comment should be after the import.

[pbackus]

Putting this import here means that it will be affected by the public: attribute specifier on line 53, turning it into a public import. This means that the symbol Int128 will be visible as BigInt.Int128 from outside the std.bigint module.

To fix this, move the import to the top of the module, outside the declaration of struct BigInt.