Tests that are solely testing constructing from a hex string
are dropped, others are modified to use a uint256 constructor
or the arith_uint256 uint64_t constructor.
Since an arith_uint256 can not be constructed from a string
directly, we need to ensure that test coverage on
UintToArith256(uint256::FromHex()) is not reduced.
uint256::FromHex() already has good test coverage, but
the test coverage on UintToArith256() and ArithToUint256()
is increased in this commit by upgrading the `conversion`
test case.
Moreover, since `uint256.h` does not have any dependencies
on `arith_uint256.h`, the conversion tests are moved to
`arith_uint256_tests.cpp` so the dependency can be cleaned
up entirely in a future commit.
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