@BioTronic In this weekend I'll post a DIP and will publish experimental patches.

@monarchdodra `BigInt` is normally expected that it having value semantics, but it owns the internal data by reference (`BigInt` has `BigUint`, and it has `BigDigit[]`). So copying mutable `BigInt` must deeply copy the internal data. It should be done by postblit.
To support `BigInt` copy conversion between distinct qualifiers (eg. mutable to immutable, const to mutable, etc), postblit is necessary.