8/10/12 bit FixedPoint numbers in the vein of Ufixed

[vchuravy]

In JuliaGraphics/ColorTypes.jl#11 the need for FixedPoint numbers with a different bit size from 32bit arises.

Ufixed is very flexible and nice for representing ColorValues, but it seems to me that its design is a bit different. Ufixed is defined from [0,1] and Fixed32 has a variable range depending on the number of fraction bits.

For my needs I wouldn't need a variable fraction bit number as all values are within [-1, 1]