`core.int128`: Add `mul` and `udivmod` overloads for 64-bit operands

These map to a single x86_64 instruction and have accordingly been optimized via inline assembly.

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import core.int128;

ulong a, b;
Cent product128 = mul(a, b);

ulong divisor64 = …;
ulong modulus64;
ulong quotient64 = udivmod(product128, divisor64, modulus64);
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