Correctly handle unsigned integers in constructor

[TZdyrski]

Currently, the AlgebraicNumber constructor gives surprising behavior when using unsigned integers. For example, AlgebraicNumber(UInt8(3)) != AlgebraicNumber(Int(3)).

This occurs because the constructor AlgebraicNumber(x) uses -x when calculating the coefficients, which causes UInt8(3) = 0x03 to be wrap around to 0xfd. Thus, the coefficients of Int(3) are [-3,1] (correct), but UInt(3) gives [253,1].

Note: while the internal representations and equality test differ, AlgebraicNumber(UInt(3)) - AlgebraicNumber(Int(3)) == 0, so testing equality by subtracting is a (slower) workaround.

This fix promotes the input of the AlgebraicNumber constructor to BigInt to avoid this issue (simply using signed(x) would not work as signed doesn't check for overflow).

[coveralls]

coverage: 92.149% (-0.4%) from 92.593%
when pulling ad32ce5 on TZdyrski:patch-1
into c8c4677 on anj1:master.