Backend: use fixed-point numerics instead of float32 for weights

[sstangl]

The backend currently represents weights using float32 values. Floating-point arithmetic and comparisons are significantly slower than integer operations and require the use of special floating-point registers.

OpenPowerlifting got a nice speed bump by using a fixed-point representation: a weight like 123.45 is stored as an i32 in the format 12345. Go appears to have an arbitrary-precision fixed-decimal representation through the decimal package, but a representation that hardcodes one decimal point (for i16) or two decimal points (for i32) is likely to be simpler and more performant.