Implement various minor optimizations in fldr_preprocess

[fsaad]

1. Remove initialization of H to -1; instead, use calloc for initalizatio nto 0 and index the samples by 1, ..., n+1; also make sure to update the sampler to check if the sample is <= n, and return one minus

https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L55
https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L97-L98

2. Remove unneeded additions
   https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L57
   https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L65

3. Change bool to int
   https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L56
   https://github.com/probcomp/fast-loaded-dice-roller/blob/2fb32e94a04c744c86aa4cb54b3ee0259894e777/src/c/fldr.c#L64

The main bottleneck in preprocessing is bit extraction from the a[i]. It might more efficient to compute the probability matrix in reverse order (i.e., with the least significant bit in the first column) and using integer division on the running values of a[i], instead of bit shifts/masking.