std.math.algebraic: Inure against integer promotion rules in abs()

[ibuclaw]

Fixes something that got missed out by #5653. FYI @WalterBright.

std/math/algebraic.d(50): Deprecation: integral promotion not done for `-cast(ushort)0u`, use '-preview=intpromote' switch or `-cast(int)(cast(ushort)0u)`
std/math/algebraic.d(59): Deprecation: integral promotion not done for `-x`, use '-preview=intpromote' switch or `-cast(int)(x)`
std/math/algebraic.d(50): Deprecation: integral promotion not done for `-cast(ubyte)0u`, use '-preview=intpromote' switch or `-cast(int)(cast(ubyte)0u)`
std/math/algebraic.d(59): Deprecation: integral promotion not done for `-x`, use '-preview=intpromote' switch or `-cast(int)(x)`
std/math/algebraic.d(50): Deprecation: integral promotion not done for `-cast(ubyte)0u`, use '-preview=intpromote' switch or `-cast(int)(cast(ubyte)0u)`
std/math/algebraic.d(59): Deprecation: integral promotion not done for `-x`, use '-preview=intpromote' switch or `-cast(int)(x)`

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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[ibuclaw]

Actually, I'll fix this by having an internal absu function in std.numeric.