Bitmap block allocator

A disk allocator manages `n_blocks` blocks via a free bitmap, all initially free. It serves contiguous allocations using first-fit. Process operations: ["alloc", size] finds the lowest starting index of a run of `size` consecutive free blocks, marks them used, and that start index is the allocation handle returned — if no such run exists the alloc fails and consumes nothing; ["free", start, size] marks the `size` blocks beginning at `start` as free (assume every free corresponds to a prior successful alloc of exactly that start/size). After all ops, return a list of the start indices returned by each successful alloc, in op order (failed allocs contribute nothing to this list). n_blocks >= 1; <= 5*10^3 ops; sizes >= 1.