Reverts llvm/llvm-project#86137
Some aarch64 compilers seem to consider that `uint128_t` is not
`is_trivially_constructible` which prevents `bit_cast`-ing.
This patch moves most of the multiprecision logic to the `multiword`
namespace and simplifies some logic in `BigInt`. It also fully
implements the mask and count functions and increases test coverage.
`math_extras.h` is also reworked to make it more concise.
